My cat has a hard lump on her back leg...?

It's hard and seems to be only attached right underneath her skin because I can pull it and the skin and the lump move and skin goes under it. She is 10 months old, healthy, has all her shots, has just been spayed, is eating and drinking normally and plays like normal. She is underweight, but has always been since she was a kitten- the vet says she was the runt.