I can remember our first cat Penny having quite a large lump under her skin on the top of her back, literally it must have come up overnight. We took her to the vet that day and the vet said it was a cyst. She said she probably had a fight with another cat and it will have got infected, she hardly went out but the vet said it can come simply from a small bite or scratch. She cut it open and let out the gunge and I had to open it and bathe it in salt water for a week. It never once bothered her - looked worse I think! It could also be a fatty lump sort of thing, Schui has one on his foot, especially common in older cats I think but as others say you'll only know when the vet checks it out, best of luck and let us know.