Lots of things jt. Thumb up or out, too much arm in the draw, head in the wrong position, poor drawing technique, etc. Also not sure what you mean by "the fat part of your thumb."
Don't mean to make light of your question but there are too many question marks in it. " Dr, it hurts when I do this." "Then don't do that."
Can you post a video of you shooting?
Arne