Accepted Answer

There is a mass called a histiocytoma that occurs in younger animals that can regress on its own. These are more common in dogs but I have seen them in cats. Insect bites are also a possiblity but I would expect rapid resolution. There is also something called Eosinophillic granulomatous complex (thats a mouthful- basically an allergy). However, I would expect to see this in a little bit older cat (> 1yr). The lesions often occur around the mouth and can respond to antibiotics or steroids. I expect this lump to be gone within a month or two, but if you have her spayed (if she is not already) you could have your vet aspirate it or removed it while she is under.
I know this is vague but the best diagnostic at this point is an aspirate of the mass. The problem is that it is an expense in something I expect to resolve. If you want peace of mind, see your ve and have it done. At least then you will be able to rest easy about it or you will be able to treat accordingly.
I hope this helps
Dr. Jeff

Expert:	Doctor Jeff
Pos. Feedback:	95.6 %
Accepts:
Answered:	9/5/2009

Cat Veterinarian

Small Animal General Practitioner and Practice owner. 8+ years of practice