Hmm.. I don't know how often you get your thyroid checked, but maybe it should be done again. It sure sounds like that's what it could be. Or, I don't know what medications you're on for the IBS or whatever else, but maybe it's a reaction to one of those? I hope your doctor can figure it out, good luck to you.
Had thyroid trouble for years but couldn't get anyone to listen to me. It was swollen behind my esophagus and could only eat food in baby food consistency but I am treated and monitored now. But my condition worsened during the undiagnosed period.
Has your thyroid ever been tested? Hyperthyroidism can cause heat intolerance. As well as fatigue, diarrhea, a racing heart, and anxiety. You should look it up and see if you have any of the other symptoms.