Every night, if I sleep on my back, I will wake up APPROXIMATELY 4 hours later very sweaty, particularly my butt and legs. No idea why. If I lay on my side, I do fine. If I lay on my stomach, I do fine.

Well, then why don't I just NOT sleep on my back?

Well, I wish I could, but I get a very sore shoulder if I do.

I use 100% cotton sheets.
I use 100% cotton clothes. The room temperature is fine (75-76)
I sleep with a sheet.
I have a fan.
It's not humid.
I'm not overweight
I have no pre-existing health conditions
I'm 20 years old

WHY?!?!