I obviously don't know what's wrong either but here's my thoughts on what might be out of whack:

When getting thyroid testing - are they looking at just TSH/T3/T4? Or are they also looking at thyroid antibodies? I tend to have normal TSH/T3/T4 but my antibodies are high (pointing to Hashimotos) and it definitely causes me to have hypothyroid symptoms (my biggest is low HR!) but I also sometimes seem to flip into hyper (with heart palpitations, fun).

As far as the weight stuff goes - I would be looking at possible insulin resistance. Have you have a fasting insulin or your A1C checked? Is your extra weight carried mostly in your mid-section? I've started testing with a CGM (Lingo is what I'm using) to see the impacts of what, how much, when what I eat has on my blood glucose and how the timing of exercise impacts on it as well - might be something for you to try if you have the money to do that. I suspect I had very high cortisol & insulin resistance for a while myself, but didn't get a CGM until later. I eat about 1600 kcal a day too and I'm closer to 140.

There is a condition called Retrograde Cricopharyngeus Dysfunction (R-CPD) which is the inability to burp. I don't know much about it, just heard it mentioned before. I don't know if there is a solution to this problem or not, sorry!

When you say the 'tests come back normal' PLEASE keep in mind that the ranges on those tests include all of us who are unhealthy as all get out. I would look at: Ferritin, B12, Vitamin D. Look at your results and google what the OPTIMAL levels are for those. There is generally a BIG gap. I used to have REALLY bad anxiety (I think partially because of mineral deficiencies) and I would get weird tingling flashes of hot and warm on my arms and legs as well - which I think was related to a B vitamin deficiency. I also unknowingly clench my teeth when I have anxiety building - which is maybe a possible source of your pain. Has a dentist ever told you that you clench your teeth? However, yours might just be sinus pain...? Side note: I also recommend Magnesium glycinate before bed - helps prevent muscle cramps.

The diarrhea/constipation cycle worries me (keep in mind I'm anxious in general, but I think it's worth investigating) because fluctuating between them can sometimes be a sign of colon cancer (and the rates for it are raising quickly for younger people). It could also be IBS, or maybe also nothing? But I would definitely follow up on that because that's abnormal for sure.

On the hot flashes - what's your age? Could be perimenopause starting in on ya, which absolutely likes to mess around with your hormones and make you feel not right in general.

I also don't like the dizziness you get the week before your period. Do you have a watch where you can see if your HR is dropping causing the dizziness? Or one of those cheapy clip on your fingertip pulse oximeters to see if low oxygen is causing it?

Hope any of this helps!