I've always noticed this weird indentation that sometimes bulges out on my mare's side, but I always thought it could be normal. After trying to research about what this could be, and not finding anything, I've decided to ask on here.
^ I circled the problem area in red.
When I touch the area, she seems really sensitive about it. I haven't noticed at what times it is indented in and what times it is bulged out. I believe both happen when she's just relaxing or eating.