Why does my stomach have a warm feeling inside?

That's either a happy contented feeling you have Amethyst or "gastric reflux" or heartburn which is causing it from eating something that disagrees with you.

Acidity  or bad food  or undigested food or there are glands inside the stomach any kind of trouble with it can cause stomach ache if its severe , then you better consult a doctor he will tell you about some tests then he will do the diagnosis from the lab reports