If you see a lump on your pig - go to a vet! Do not play the "wait and see" game with your pig.
In May of this year, my elder boar went off his food and generally behaved as though he was sick. I rushed him to the vet. There were no neon signs to direct us. His teeth were checked, and we had bloodwork done. His white cell count was a little high, but we took that to mean he had an infection. He was given Baytril and Critical Care and within a week was back to his usual nonsense.
In August, I noticed a lump on his inner thigh. Max had always walked a little funny (high on his toes) so if it had effected his walking previously, it was not enough to notice.
The first vet I had seen could not see him right away, so I was referred to another vet. This second vet took a sample of the tumor via a syringe and sent it off to a lab for a cytology report. The first time she tried, she got nothing but blood. It was that close to the vein.
She told me that while rats usually have benign tumors, and gerbils usually have malignant ones, guinea pigs are 50-50. We discussed his age (4.5 yrs) and the unfortunate location of the tumor. Being that he was older and the tumor was right on the main vein to his leg, surgery was probably not a good option.
I went home and waited for the results. Someone on CG told me that Peter Gurney thinks that surgery is not always the best thing for cancer as in some cases it seems to spread the cells.
The results were that it was a malignant epethilial (I think - hard to remember the technical term) or skin-related cancer. We again discussed treatment and realistically, all I could do was keep him comforatable. We didn't think he'd survive surgery.
I put a divider in when it seemed that Steve was pushing him around and tried to feed him anything he would eat. But it was obvious he was slowing down. It was like "okay, you know I'm sick. I don't have to hide it anymore."
A little under two weeks later, it was obvious he was suffering. He just stopped. I made the decision to let him go.
It was very fast and very sudden. In my case, I don't know if we would have been able to do anything differently if I'd known earlier. But my advice to anyone else that finds a bump on their pig is not to mess around with it. If it's a cyst or an abcess, you want it treated. If it's a tumor, you want to know before it gets too big or attaches to a vein/spreads/becomes inoperatable.