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Hello...I'm Dr. Coldiron and I will attempt to help you with your veterinary question! Lumps in female guinea pigs can be due to multiple things. If internal, I would question cystic ovaries or abdominal mass. If more external (confined to the upper layers of the skin) I would think cancerous or non-cancerous mass or abscess. Because this mass has shrunk recently, it makes me think more of an abscess that would have resulted from some type of puncture to the skin (bite from another pet, etc.). The mass should be lanced and drained and then flushed to remove bacteria etc. Antibiotics would be given for about 2 weeks. These are not usually very problematic! Have someone check the mass to see if is an abscess...if so get it treated. If its a mass, have it removed...find someone with a good eletrosurgical unit or laser to help prevent extensive blood loss.
Has your veterinarian performed a Fine Needle Aspirate (FNA) of the mass? This would indicate if the mass was an abscess or tumor of some type. This is a mass and not cystic ovaries or a gravid (pregnant) female, correct? Could be an abscess that is not responding to the antibiotic treatment. In most cases, an abscess needs to be drained and cleaned out and then antibiotics used orally. See what your vet found on aspiration of the mass.