Does it look like gold or rust colored "dust" when you shine a light on her, especially around her gills? If so, that is velvet, if not, something else is going on, like maybe stress of new surroundings or shock of new water conditions if you didn't acclimate her properly. Just a reminder that Bettas are tropical and need a constant temp between 76 - 82 all the time.
For Velvet, you need darkness (put a blanket over her bowl) and warm, clean water in addition to the treatment. Velvet is photosensitive, the warm water speeds up the life cycle causing it to drop off the fish quicker, the dark keeps it from reproducing and reattaching to the fish, and the medicine kills it off.