The most common cause of urination of blood is a Urinary Tract Infection – or UTI.
If left untreated this will develop and can kill a guinea pig. Usually at the point of excreting blood, the guinea pig is in extreme pain when urinating. You will notice a high pitched squeal and hunched movement if this is the case.
Your guinea pig will need confirmation of this by the vet who will then prescribe an antibiotic which will sort out the infection. One of the indicating signs that this could be the problem is a wet or damp bottom area as the piggie tends to urinate more frequently.
Once you have seen a vet you can also give your guinea pig cranberry juice (diluted into their water) or Barley water – which both help reduce pain and symptoms of urinary tract infections.
Bladder or Kidney Stones
Can be another reason that you are seeing blood. Sometimes you can have a UTI occuring alongside bladder issues, and in both cases immediate vet care is required for treatment with antibiotics and potentially a scan to detect stones.