I need help!!!?

I getting a kitten sometime in the next two weeks from a friend of mine whos cat had kittens. The kitten that i am getting is the runt of the litter, and the mother feeds her but does not lean her.. Could any one tell me why? Also she has a lump on her that the "for now " owner is washing and she said it was going down and getting better, but when my little Gracie walks,(yes i named her already) she hobbles because it bothers her. She scoots kinda, and lifts her back legs up . Is there something terribly wrong? The "for now" owner said that her mom had that too, and it got better. But i am very worried.. She still looks like she was just born.. (size wise) and she doesnt seem to be growing.. Could some one tell me what might be the problem, and if she is gonna make it? She is 5 weeks, going on 6 on May 1st.