DEFINITELY have it checked by a vet. One of the worst things about cats is that an alarmingly high percentage of lumps found on them either are or can become cancerous if not taken care of. Is she spayed? If so, then the concern about mammary cancer is somewhat decreased, depending upon the age at which she was spayed, but it should still be checked out. There is any number of things that it could be, some of which need veterinary attention, some of which don't. But only your vet can tell you for sure. Good luck! Please let us know what happens!
Ghilly
It could be an abcess forming, in which case it should get bigger and possibly drain out on it's own. Ideally, if you can, you should take her to the vet (as ZodiacQueen suggests) for antibiotics if it's an abcess.
If it doesn't go away or change then it is some kind of cyst or growth that is not necessarily malignant, but should be checked and possibly biopsied.
Let us know what happens and if we can help in any way.
Not to scare you, but ANY lump should be checked. There's a chance that this could be breast cancer. Cats that are at higher risk for breast cancer have either not been spayed or have been spayed AFTER their first heat.
Good luck to you and your kitty.