Yeah, she's thin. I got one from [insert name of reputable breeder] that was 6 months old and thinner than that, and all she's done since I got her is eat. She eats seconds and thirds and sometimes chomps me when I go to take her out of her feeding box because she's hoping my hand is the 4th helping.
Ball pythons can be sexed by a qualified vet or breeder at any age. It's really only tiny babies like green tree python hatchlings that can't be sexed until they're larger.
(Oh--just saw that this is an older thread. Glad she's a girl! Keep feeding her, especially if she acts hungry.)