there's a lot of things it could be. a tumor's one of them, but that's no reason to jump to conclusions
If he doesn't seem to flinch or react when you touch it, it could be nothing. Keep an eye on it for the next few days and feel him down to make sure you don't find any more. If you find another or he starts acting ill, take him to the vet. Not much they can do for kitty cancer unless you have boocoos of money for chemo and surgeries (which still can't and don't guarantee anything), but it would be nice to know if he was sick and suffering so you could do the right thing by him.
Naw, if you feel a little lump on an animal's belly, it obviously means cancer. And after jumping straight to that conclusion, you must just as surely jump to the solution. Is it dead yet?
I would suggest perform your own operation on it and remove it. Be sure to use a sharp kitchen knife!
damn it, i wanted to wait a few more years before we had this talk... now eskimo, that lump means that it's a boy cat...