You may have Gildert's Syndrome. My GF was diagnosed with this in the last two years, one of the side effects is you constantly feel warm as your metabolism is on constant burn.
She can eat 5000 cals per day and not put any weight on, she's sick of being skinny 9.5 Stone and 5 foot 6 inches.
She's even tried the hi calorie shakes from the GP ( given to people with cancer and people who've had organ transplants) and it still doesn't work.
I myself am the opposite, I've just got to look at a chocolate bar and I put a stone on.
!'m sitting at 16s 7ilbs, I've lost 16 pounds in one month from terminating all bread,cakes and proceesed/rifined food.
I eat one baked potato per day with 3 tablespoons of beans and a few dollops of Mayo and I'm done for the rest of the day.
It's took me ten years to find something that works for me. But I was a former body builder. The weight is dropping off me now, I can only hope I reach my target weight for Yule.