Suggestions needed - actual TDEE much lower than expected?

TrishDLish81

A little background: I've been on MFP since the end of May, about 9 weeks (I know this profile says I joined in July; I like to keep my forum postings separate from where I log my food because of some IRL people on my friends list). I started out at 237lbs, with a BMI of 38.2 and a BFP of about 50%. I will fully admit that prior to getting started on MFP, I spent far too much time on my tukas and was eating far too much crap.

When I first started, I set my goals to a 750 calorie deficit per day (on sedentary) and bought a FitBit to monitor daily activity and a HRM to monitor workout calories. I started out by eating back maybe half of my exercise/FitBit calories, about 1600-1700 calories per day, with just a couple of exceptions for "cheat days"/special occasions, where I still stayed well under my TDEE. I lost almost 10lbs in 4 weeks, then over the next 3 weeks actually gained a pound and an inch on my waist. So, after reading some of the "eat more to weigh less" stuff on here, I decided to give that a try. So, I started eating ALL of my exercise/FitBit calories per day (about another 200-300 calories per day), which put me really close to calculated TDEE -30% on average, and maintained my activity levels and workouts (some cardio, some resistance training, 3-4 days per week) for the next 2 weeks. I gained back another 4lbs over that 2 weeks (I'm too scared to take measurements). These are not just water weight fluctuations - I weigh every day to note trends, and it's a consistent, steady gain. I can't open my diary because I don't log on this account, like I said, but my macros are set to 40/30/30 (though, admittedly, I often go over on carbs by a small amount), and while I do eat some junk here and there the bulk of my diet is made up of fruits and veggies, healthy fats, lean proteins, and whole grains. My one place where I really seem to fail despite my best efforts is sodium, which I know can cause water retention, but if I'm consistently eating 4,000mg of sodium every day, even if I'm holding on to a few pounds of water it should be a consistent few pounds and my fat loss should occur around that, right?

I actually went back through my diary and averaged out all of my calories eaten vs my calories burned per my FitBit and HRM, and I've maintained an average daily deficit of 832 calories, which over 9 weeks theoretically would put me at about a 14-15lb loss, right? I'm down less than 5lbs in that time. For someone with about 85lbs to lose, this seems unacceptable. I'm weighing and measuring and logging every single bite I'm eating (at least, whenever possible, but I make 90% or more of my food at home and I've failed at weight loss too many times to lie to myself in my diary) so I feel confident that my calories in are as close to correct as anyone can hope to be. Obviously my calories out are where the problem is. My body is actually burning far less than it "should" be based on my size and activity level. I've compared what my FitBit and HRM are telling me to every online calculator I can find, and I'm getting really similar results on what my body "should" be doing - I don't think I'm counting or calculating anything incorrectly. My weight loss is just not reflecting it at all.

Has anyone encountered something like this? What reasons could there be for this to be happening? All I can find on the Googles is a potential thyroid issue, but I had my thyroid tested about 2 years ago when I had similar problems and was told that my results were normal (TSH was 3.57 and T4 Free was 1.07, if that means anything to anyone). I put my weight loss efforts on hold shortly after that because I got pregnant and then was nursing, and now here I am trying again with the same exact problems. Is maintaining a huge calorie deficit (at least based on calculations) my only hope? Am I doing something completely wrong and just not realizing it? Any other suggestions for something that can get my weight loss moving? I'm really open to any ideas or criticisms right now.

TL;DR version: Calories in vs calories out isn't working for me. Help?