Relating this to my dog, sometimes animals have mysterious bumps that appear, and most are benign.
Especially in an older animal, whatever they are, tend to grow slower and the vet will just let it go -- but the vet would also tell you if it starts growing quicker to come in.
Then there are (certain types) of bumps that simply get drained of fluid, in the vet's office.
So in the case of your cat, since this is something new and uncommon, you might want to consider a vet visit (no rush), if for nothing else, your own peace of mind. Again, the age of the cat will be taken into consideration (probably more of an issue if it was a young cat, for example, due to a higher metabolism--leading to a more rapid growth of any lumps).