There are several reasons why a guinea pig might bleed, all of which warrant a trip to the vet. For example, she could have a UTI, bladder stones, or something going on with her lady parts (check out Glynis' thread "Velvet has been bleeding "). No matter what the problem is, she needs to be seen by a cavy savvy vet. And yes, guinea pigs can get mites. Do you suspect that she has some?