My cat is sick i need answers from people who knows about cats/animals?

my cat gave birth like 1 or two months ago i don't remember exactly when.anyways my cat gave birth to 1 kitten and that kitten came out healthy.the kitten been drinking milk from the cat. i gave that kitten to my brother because the cat is already grown.anyways i gave the cat to my brother 2 days ago and just today i noticed on the mother that the area were the kitten was drinking milk is like hard.by hard i mean like it looks swollen i don't know exactly.does anyone knows what is that?i am scared because that never happen to the cat before..do i need to take the cat to the animal hospital or something?is it serious?and please if you know about animals please answer.