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Guinea pig health is a steep learning curve. Does your piggy have dry skin on her nipples? That can be a sign of hormone problems. How old is she? If you can take a fresh urine sample to the vet, that could be a useful diagnostic test. If you put her in a plastic base carrier, after a few minutes she should pass urine. If you can extract it quickly to avoid contamination that is best. Does she have any large lumps in her abdomen? Is she vocalising pain or hunching over when urinating? I hope the problem can be solved

Keep going back to your vet if she is still passing blood.