As she's a livebearer and was with some males at the LFS, she's probably pregnant, especially if she's eating and otherwise acting normal (she will slow down a bit when very near to birth). It's a bit hard to tell from your picture and also because she's black. I don't know if this would help, but here is a picture of 2 of my guppies, the bottom one is not visibly pregnant, the top one is. A pregnant molly would look similar.