I have seen mollys and platys with odd behaviors like this before birth. From the pic, she looks very pregnant. I had a molly who before birth, stopped swimming, she layed on the bottom and let the filter suck her in, she laid sucked against the filter for a day before giving birth, she then swam to the surface for food and lived happily for 3 years, she did this every so often during births. However the tanks ammonia ntrite and nitrate levels are helpful in finding the true problem. Also if she does have a bacterial infection, tell us what it looks like and where it is on her.