Sorry to hear about your problem.
In my experience there could be a variety of reasons for the bleeding.
How old is your piggie? Is she squeaking at all when she is having a pee? Is she sore when you feel her abdomen, can you feel any lumps?
I'm pleased she is otherwise quite happy, but has she lost weight recently or shown any odd behaviour eg towards her companions.
It's important you take her to your vet with all this information as they can check to see if it is related to a urine infection, perhaps a bladder stone or cystic ovary (x-rays or scans may be needed).
My first thought would be that they would probably give her a course of antibiotics (septrin or baytril) but be aware that this may only be a temporary soution and be prepared that the bleeding may return when the course is over and further treatment may be needed.
If you are in any doubt about the confidence of your vet with guinea pigs or they seem at all uncertain about the treament, or suggest surgery, then try and persuade them to contact maryh's vet to ensure they consider all the other options.
There are similar threads on the forum and, although the outcome was not good on all of them, this was more down to fact that "treatment-wise" things could have been done differently.
Hope you get on OK