Hi!
Good that you have booked in Stardust. Like any lump, it needs to be vet checked because it is not just about the location but also about the consistence; lumps can be anything from harmless cysts or fatty lumps to abscesses or tumors (benign or malign).
Without any pictures, we can really only guess and wild guesses won't help you any. We can mention that mammary tumors can happen in both genders and - as long as they are removed early enough - are usually not a problem. But we cannot tell in any way whether that is what you are dealing with or not.
Guinea Lynx :: Lumps
All the best.