Guinea pig has a tiny bit of blood on butthole

My male guinea pig who is 5 had a tiny bit of blood on his butthole this morning. I noticed cause he made a bit of noise while I'm guessing while he tried to poop. My boy has a benign tumor behind a testicle, so it does press up against his butt if that makes sense? But it doesn't appear to have grown.

He is eating and drinking normally. Still very enthusiastic about food and is running on the ground like always.

I don't have money to treat something like bladder stones, so if it's something serious, I'd have to put him down.

Did he possibly nick his bum against some hay? Constipation? Though he seems to poop just fine, no bloody stools from what I can see. The spot of blood on his bed was pretty tiny, like a smear rather than a dripping source.