Well there you go. You got her from petsmart, it's going to take awhile to get her nice and chubby. I would keep an eye on her for parasites. If she doesn't start gaining weight in a couple months I'd take her to the vet.
Also, ball pythons don't need vitamins. That stuff is junk and that lady works at a pet store. If the pet store can trick people into buying things they don't need, they will.