Many times , live bearers such as your black molly are bought from pet stores already pregnant. They can store packets of sperm long after they have first been inseminated so they do not need to be near a male for quite a while, yet can still become pregnant.
I would keep an eye on here, looking for behavior like staying in a secluded corner of the aquarium, and eating aggressively or not eating at all. Keep in my that the fish may eat her fry (Babies) if given the chance, so keeping floating plants is recommend. Any More questions can be asked at the link in my sources.