My Guinea Pig is 4 years old now and almost 5. She is really happy and still acts like a young one:) But I noticed she has Big calluses and then there are like holes in bot feet and poo gets in them. Is this bad. The vet said to use lotion but it doesnt look like it's making a difference.

I used to have a Guinea pig and he had calluses on the bottom of his feet too! He was a rescued guinea pig so I got him with his feet already like that. I think it might have to do with him wetting his bedding. Be sure to change his bedding often so that they aren't on the moisturized cotton stuff. The calluses are most likely normal (just think of a human not ever getting a pedicure, and how we develop that thick type of skin on the bottom of our feet), just keep an eye on them, that they aren't getting infected.

Just keep putting the lotion on it and it should soften so you can remove most of all of it with out harming the guinea pigs. Most guinea pigs get them as they do not run on a rough enough surface to ware it off as it would if they were in the wild.

32 years dedicated to caring for and loving guinea pigs

What floor is she on? If she is on a wire cage you need to cover the wire.
Is it spurs?
http://www.guinealynx.info/feet.html
Was the vet you went to cavy savvy? You need to find a guinea pig vet.