What Breed Is My Dog?

We were told she is mixed with a Pit, but we do not with what. She was taken from her mom at 3 weeks and we got her at 6 weeks and 3lbs. She is now 12 weeks and 13lbs. Unfortunately we didn't rescue her through a shelter, but we were able to rescue her from Boston. She is very energetic, but bites hands and feet when excited. We are new puppy owners, but are willing to take on the challenge and give her the best life imagineable. We just wish we knew what she was mixed with and our vet has no idea. The vet said she will most likely be 25-30 pounds when fully grown and that her malnurishment as a pup and being taken from her mom so early could have contributed to her growth. Any suggestions on what she could be a mix of?