inability to gain or maintain weight

I am 28 years old , male 5' 9.5'', I was always on the slim/muscular side. I use to be 147 pounds with 5.7% body fat.
something happened about 4-5 months ago, I started losing weight and muscle fast. I am underweight now and have lost close to 20 pounds! I look like a holocaust victim. I will stuff my face full of food and get a pot belly and then by the time I wake up in the morning the pot belly is gone and instead the belly has sunken inwards and my pelvis bones are sticking out. Ive had insulin and thyroid checked and they are fine. I have no idea whats going on. I noticed my stools are not formed well and are not consistent, they are always different. Ive had a scope done too and its normal. I have no idea whats happening. It seems like something has sped up my metabolism super fast. I do not drink caffeine, smoke or do drugs. I am not on any medications. I dont know what else to do. Can someone help me please?  Ive lost weight all over my body and im very desperate to find out why this is happening and how I can fix this and how I can find out whats wrong.

on a side note, since this has happened ive had terrible nasal congestion and blockage and drainage on a daily basis all day long. I dont know if that has something to do with this. Its almost as if as soon as the food gets in my stomach ,. in little or no time at all , it gets burned off. its IN and OUT. I dont know if i have a malabsorption issue or what.

other than thyroid and insulin, what else could raise the level of metabolism? and what kind of malabsorbtion diseases / conditions are there?

thanks for your help

[I_HATE_ANXIETY]

I forgot to add my list of symptoms which I encounter on a daily basis, sometimes they are severe enough to make me go to the ER where I have been many many times , they dont do much over there except check vitals and tell me to see family doc. Family doc is stumped as am I. I do not get these symptoms all at the same time and they differ in severity from day to day.

- Lightheadedness
- Burning / "spicy" / hot sensations felt in skin, head, nasal passages, throat, face, ears, hands, arms, eyes
- palpitations (hard pounding heartbeat felt in hands, fingers, chest, neck, feet)
- muscle twitching in mid-chest and abdomen area
- muscle pulling/stretching in chest and upper abdomen area
- chronic nasal congestion and drainage(clear, glue like, dries hard)
- loss of muscle mass / loss of weight (loss of 20 pounds)
- inability to keep or maintain weight
- weakness in legs, hands and arms
- headache
- soreness/pain in nape of neck/base of skull
- chest pains (sharp, aching, stabbing)
- excessive gas (burping and flatulance)
- bloodshot eyes