Gaining weight on 1200 calories HELP!!

Are you weighing all of your food on a food scale? Have you started a new exercise routine? Did you happen to eat a lot of sodium yesterday? If you truly ate 1200 calories a day all week there is a 0% chance you've gained any fat whatsoever.

If you are new to exercise, your muscle will be retaining water to aid in recovery and protect them from the new exercise.

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

I’m not going to the gym or anything but we have two mad Border Collies who need a lot of walking so doing 3 miles a night. I know it’s early days and week isn’t very long and hormones make a massive difference. I’m hoping that’s what it is and not just that I’m destined to get fatter and fatter 😂

What pace do you walk the dogs at? Are you getting your heart rate up or is it a more leisurely pace?

My impression is that, assuming you are overweight, that you are probably underfueling for your lifestyle. Walking a continuous 3 miles at variable incline and pace every day is not a sedentary lifestyle. You might have a sedentary job, but you are regularly exercising.
Also, it is my opinion (obviously) that you shouldn't use something like thyroid issues to do the bare minimum calorie intake of 1200 unless your doctor expressly requests you do so and you are having your vitamins and other nutrient profile factors regularly checked.

You don't indicate in what time frame this increase was. If it's 4lbs in one week, it's almost guaranteed to be water retention because you can't organically gain 4lbs of fat on a 1k+ deficit in one week. It's just not going to happen. I mean, unless you're like, SUPER constipated.

Frankly, I wouldn't recommend eating on the extreme side as you're describing without supervision unless you're prepared to research a lot and accept that any results will not be linear. If I were you, I'd quit doing this guessing at an extreme caloric intake diet and opt, instead, for a more robust level of intake and focus on a lifestyle change rather than the scale. The scale victories are nice, but the non-scale victories are what safe your life.

Also, food for thought... You may have hypothyroidism, subclinical or otherwise, but if you're on the proper therapeutic level of hormones, your metabolism should be relatively "normal" and need minimal accommodation. You could be causing some unnecessary secondary issues.

Try weighing only once a month at first if you can't manage the feels that go with fluctuating from day to day... You could even discuss with your physician a desire to build a healthy diet and exercise program for your medical condition and request weigh-ins every month, and simply focus on non-scale stuff other than that. That way, you can get the input from them regarding how much you should eat, what you should eat, any supplements you might need, etc.

Please don’t let the scale stop you. I’m also on a 1200-1300 calorie meal plan. The scale can be tricky, why? I don’t know. I made a conscious effort to track my weight daily because I’ve let the scale hinder my progress many times in the past. My weight can fluctuate up to a 5 lb increase in one day despite sticking to my plan and exercising. Recently I decided to take a day to increase my activity and do some extra cardio and strength training. I burned a ton of calories and the scale went through the roof the next day. It’s probably water retention or something else, but I’m certain it’s not fat gain or from an increase of calories. It’s just the body adjusting the way that it does. I still lost 9 lbs at the end of my first month. 1 lb more than I expected. Life and weight loss is a journey, focus on the progress you make in doing what you know you should and being consistent. The scale will reflect that in time.