It's why he's been so cranky lately though. And he's going to the vet in the morning for further examination, first an ultrasound to see what it is, then a surgery if possible.
No fun for kitty, but with luck he will be able to recover for a while.
Update: The cancer is a fairly large tumor which has surrounded several loops of his small intestine. It is touching but not invading his liver, and spleen. Surgery to remove bulk from the tumor would be too dangerous. It's still (apparently) in stage 2, which means chemotherapy is his only hope for survival, and I've signed up to take him in tomorrow for a further examination by the same doctor who did Vinnie's thyroid treatment. They'll do a more detailed ultrasound and decide what kind and how much chemo to give.
Update the second: I didn't update this Thursday because I was trying to get work done and I noted it on my facebook.
Clint has carcinomatosis, which is a scattered, overgrown carcinoma in the abdominal cavity.
It cannot be removed surgically because it consists of not just the large tumor (which is wrapped around a couple loops of intestine) but also because it consists of many hundreds of tiny carcinomas everywhere elsewhere. It cannot be treated with chemotherapy effectively. Radiation would kill the rest of his organs along with the cancer.
I've been given some prednisolone to help reduce the painful inflammation, and a prognosis of a few weeks tops. When he gets to the point where he won't eat or drink and won't move around, then I will take him to the vet so they can ease his passing, rather than letting him sit around slowly starving to death.