What are these lumps on my kitten's neck?

by Maria
(California)

Can anybody tell me what these lumps on my kitten's neck mean? I found a box with two kittens (a boy and a girl) left in it with half a small bag of adult cat food. I've never had cats before but I didn't want to leave them there.

Both cats seemed normal at first and shy and timid, then got comfortable and were playing around and eating lots of food (kitten food). Then out of nowhere one of the kittens (the boy) starts acting very lethargic and stops playing with the other kitten. He was just being beaten up by his sister, so I separated them.

He was doing better for a little while but now he doesn't play at all or even walk around. He's eating less and now he has a huge lump on the front of his neck and his head.

He's very sweet and doesn't seem to be in pain when I touch it or him. It still just seems very random and only affected one kitten. They can't be more than 3 or 4 months old and I got them about a month ago.