Why is my guinea pig having fits and has runny poo with blood in it?

My guinea pig began having this problem and I highly recommend going to see a vet. It is very possible that it is a UTI which can be cured with some medicine and possible pain medication depending on how much pain your piggie is in. Even if this isn't the problem it is VERY important to contact your vet because what ever is causing the problem may worsen. If it is a UTI and you don't see a vet it can lead to stones and these can require surgery so its better to help them ahead of time. The vet may also tell you that you are putting to much vitamin C in their diet and if you ask they can give you a list of some foods that are better for them.