Started Very Active Job - Balancing Weight Loss

Look at your Fitbit graphs for the day and see if it automatically slipped into workout mode for chunks or big blocks of time.
It might have with enough steps and HR increase that likely happened.

If that is the case - it went from normal daily activity distance-based calorie burn (which might have been under or over estimated with so much upper body movement), to a HR-based calorie burn (which would have been inflated estimated since out of shape and formula not a decent estimate at lower end of aerobic range).

What kind of distance did you get for the day and total steps?
Because distance and mass and time is a very good calc of calories burned.

But if a lot of bogus steps (likely small distance at least), or your stride-length is off - you may have inflated distance and therefore calorie burn even if it was using the best correct method.

Hey, an active job will both help you get into better shape and make you glad you did it. Double win!

(As I stand here at my sit/stand desk...)

Apple Watch supposedly “learns” your habits for accuracy.

I know nothing about FitBit.

But I’m wondering if they use a similar algorithm and you have a sudden change in activity level, if the device isn’t utterly confused, or could even use a re-set?

I plateaued when I used Fitbit and ate back my exercise calories for 2 years. What I found was that to lose weight I had to up my daily calories while active to 1700 and not eat back exercise calories. I think that's why so many people say only eat back 25% of what Fitbit gives you.

Devices can over-estimate calorie burn, but they can also under-estimate calorie burn. Ditto for the so-called calculators (like MFP's) that give us our starting point calorie estimates.

Heybales is a really good advisor on questions about how devices estimate calories, IMO. I'd pay attention to him on that subject (among others). (No, I don't know him IRL, but I've read his posts here for years now.)

Still how you feel (like you ran a marathon, you said) isn't really a great indicator of calorie burn, even though it's normal to feel like it must be. It's mostly an indicator of whether we did something that was quite a challenge for our current fitness level. As a generality, the exact same activity would feel much easier to a very fit person of the same size, but (for most activities) it would burn about the same number of calories. A heart-rate estimate for the two people might even give different calorie estimates for the same activity, because the fit person's heart would beat slower . . . but devices estimate calorie burn, they don't measure it.

If I were you, I'd want to keep track of the daily Fitbit estimates for several weeks, plus keep careful logs of your eating, and see what the loss rate is after at least one full menstrual cycle so you can compare bodyweight at the same relative point in at least two different cycles. (You look to be of an age where that would apply; if not, go for 4-6 weeks.) Along the way, either set yourself to very active on MFP, then add any other (non-job) exercise calories; or use heybales' recommendation for estimating the walking part of the job . . . then, at the slightest hint of otherwise unexplained fatigue or weakness, or very fast loss after a couple of weeks, eat a bit more. Undereating, especially with greatly increased activity, is risky, health-wise. Not losing fast enough can have its own risks if at a high body weight, of course, but slowing a little bit for 4-6 weeks while handling the physical stresses of that new job shouldn't materially hamper the reduction of health risks you get from weight loss.

Let us know how things, work out for you, OK? Hoping you find a route to balance sustaining your energy at a high level, get the activity benefits from your new job, and keep a sensible weight loss rate in the picture. Cheering for you!