If you have a fish that's not behaving normally, the first thing you should do is a large (50 or 60%) water change, with warm dechlorinated water.
I'm afraid your pic's too small for me to even guess what's going on; it could be that she's giving birth (watch her carefully for a while; you don't really want to water change when she's right in the middle of labour), or it could be that she has intestinal parasites or a hernia or prolapse of some kind.