Kittens skin turning bluish

Question
I just want to know if I should be concerned or not. There are 4 stray kittens that have been hanging around my house, they are about three months old( I know this because the mother attempted to have them on my door step but I found a more suitable space for her), and two days ago they were all fine, yesterday one shown up with a limp and would cry out when her hip had pressure on it from one side but later seemed fine and running on it. Today she seems to be in more pain than before, she doesn't really move much and when she tries she cries out like she were being attacked, and the skin on the inside of her thigh of the injured leg/hip area has a bluish tint to it. I wonder if it may be internal bleeding but I honestly don't even know how she got hurt.

Answer
She most likely got into a fight with another cat and got bit. She is going to develop an abscesses there and is going to need antibiotics. You can take her with the kittens in a carrier to the vet and have him look at it to be sure.
She should also be vaccinated so that she can pass some immunities onto these kittens.
I hope she and her kittens do well.