I have two adult guinea pigs, however only one seems to have problems. The one guinea pig has swollen back feet, the front feet are fine. I don't know what's causing it, I hadn't witnessed anything that could've made it happen and I don't know what did it. He also has super small eyes, they used to be normal but they've gotten really small. He doesn't move a lot because of his feet, and when he tries to walk it looks very painful. I'm also confused because only one guinea pig is having problems, the other seems fine. I'm super worried and I don't know what to do, could anyone tell me how to treat him and what the problem is?