Weight loss slower when lifting weight?

It is not muscle gain...that doesn't happen that quickly and will not outpace fat loss. New exercise or increasing intensity and/or duration of existing exercise causes water retention that aids in repair of broken down tissue. Excessive amounts of exercise can also cause water retention due to overly stressing out the body and raising cortisol levels. 1800 Calories per day also seem pretty low for that much exercise which can also put a large stress on the body raising cortisol levels.

With only 20 Lbs to lose, 10 Lbs per month is very aggressive. Were you exercising this aggressively in prior attempts? If you've previously lost 10 lbs per month on 1800 calories with little or no exercise, that means your maintenance calories without exercise is in the neighborhood of 3000...so you've already taken a cut of around 1200 calories from maintenance with diet alone and little to no exercise...adding in a ton of exercise is compounding that deficit.

That is not only a big stress on the body which is going to jack with hormones, it is also going to set off alarm bells within your body that you aren't getting adequate energy (calories) and that will slow down some basic processes like growing hair or nails, make you more lethargic in your day to day...less fidgeting, loss of menstrual cycle (females obviously), etc. Essentially your body downshifting energy expenditure in your day to day to compensate for a massive energy deficit. Merely existing expends a lot of energy and ultimately the body will shut down or slow down what it considers to be non-essential functions to conserve energy.

Possibly water retention. For me, I gain a couple of pounds of water weight when I resume regular weight training (it's seasonal, for me). I tend to hang onto it until I stop again in Spring. Other people report quicker shifts, like gain the day after strength training (rest day), drop the following day. Others are like me, it hangs around.

As an aside, losing 10 pounds a month, if only around 20 pounds of excess weight, is not something I'd encourage. Researchers suggest that we can only metabolize a certain amount of stored body fat per day, per pound of fat we have. (There are differences of opinion on how much that is, exactly; may vary by individual?)

The implication is that losing slower may better preserve lean tissue (including muscle) alongside fat loss. Oversimplifying, if the body needs to compensate for a calorie deficit, it's going to burn body fat sometime during the day, but if it reaches limits, it can burn other tissue.

On top of that, fast loss is a stressor, cumulative with other stresses in our lives, both physical and psychological ones. Combining calorie deficit with a big ramp-up in exercise increases the stress. High stress can increase water retention, hiding fat loss on the scale, encouraging some people to charge harder at deficit/exercise.

Not only is that a way to spiral toward a crash and burn, quite possibly before reaching goal, but it has consequences for other physical systems. Personally, right now, I really value a strong immune system, for example.

I'm a fan of the "0.5-1% of current body weight weekly, maximum" as a rule of thumb, with a bias toward the lower end of that for anyone not severely obese and under close medical supervision.

One thing for certain: In a month, muscle gain is not a major factor. First, fat loss and muscle gain are at cross-purposes: Best muscle gain happens with extra calories, not in a calorie deficit. Loosely, the bigger the deficit, the less likely is muscle mass gain.

On top of that, muscle gain is always relatively slow. Under the best conditions, a pound a month of muscle mass gain would be a really good result for a woman (maybe 2 pounds for a man). Best conditions would be a calorie surplus, a good progressive weight training program faithfully performed, excellent nutrition (especially but not exclusively added protein), relative youth, and being male (it's a hormone thing).

Yes, one can gain muscle mass in a calorie deficit, especially if new to strength training, but neuromuscular adaptation (NMA, a.k.a. CNS - central nervous system - adaptation) dominates at first. loosely. That's better recruiting and using muscle fibers we already have. Sometimes people think they've gained mass quickly because they've gotten much stronger (from NMA), may look a little more defined (water retention "pump" in the muscles for muscle repair, maybe with loss of a bit of the overlying fat layer that was hiding existing muscles). It can be deceptive.

So, gaining a pound or two of muscle mass would be an excellent pace, and it's highly unlikely in a calorie deficit. On the flip side, half a pound a week of fat loss is about the slowest loss rate that most people might find satisfying, and even that can take weeks to show clearly on the scale, amongst routine daily multi-pound water weight and digestive contents fluctuations that are part of having a healthy body. (A weight trending app may help, but even those can deceive, sometimes.)

The point is that no reasonably expected rate of muscle gain is likely to outpace any reasonable observable rate of fat loss on the scale.

Eating too few calories can be counterproductive in a variety of ways: Health risk, fatigue that causes subtle or major reductions in daily life calorie expenditure (not so much exercise, though intensity may suffer, but just doing less moving at home, work, by spontaneous movement like fidgeting, slightly reduced body temp, slower hair growth, etc.). Are you losing too little from eating too little? Improbable IMO, this early in the process. The adaptations are likely to take time to kick in.

Much more probable: Logging issues (it's a skill that takes time to develop, can be subtle - you don't say whether you've used the close counting technique extensively before) or water retention.

On the latter, this is a good read:

https://physiqonomics.com/the-weird-and-highly-annoying-world-of-scale-weight-and-fluctuations

One final comment: To lose 10 pounds a month, you'd roughly be looking to lose 2.5 pounds a week, which equates to have a 1250 calorie deficit daily, i.e., eat that many fewer calories than you burn every day, on average.

The average woman probably burns something in the 2000-ish calorie range, though less for some, more for others. Exercise can increase that a little, but less than many people imagine. By eating 1850 calories a day, expecting to lose 10 pounds a month, you're estimating your TDEE (total daily energy expenditure) to be 3100 calories (to maintain current weight). That's possible . . . but IMO very improbable for most women, even with exercise. (Maybe with a super active job, plus exercise?)

Take a look at a TDEE calculator, to get a rough estimate of your TDEE including exercise. This is a pretty decent one, in that it lets you compare multiple research-based formulas, has lots of activity levels with descriptions that at least hint both job/chores and exercise matter:

https://www.sailrabbit.com/bmr/

Maybe you'll see 3100 as your estimate, but that's pretty high for a woman, even a reasonably active one, with only 20 pounds to lose.

Is it possible that when you used to lose 10 pounds a month at this calorie level, you were more active on your job, at home, with hobbies/recreation, etc.? Lots of us simplify those as we age, down-regulate TDEE a bit that way. Some of us are doing less because of the pandemic, like working from home instead of getting incidental movement commuting, moving around an office, etc.

If I were you, I'd take a look at some of the links above, give it a think. You may simply have found a good calorie level for sensibly moderate loss under current conditions. I'd lose at 1850 (gross intake, i.e., not eating back exercise), though probably only around a pound a week, but I don't have 20 pounds to lose (and I am quite active exercise-wise).

Best wishes!