Hello, I am Dr. Andrea Russo and I am happy to answer your questions about Lucy.
I have a few questions about the lump - Is it on her body behind the shoulder ? Is it soft and can you place your fingers around it and move it around? Does it feel squishy like it might have fluid in it? It certainly may be a cyst or a fatty lump but they aren't that common in young cats. I would be more concerned if it feels like a hard rock and seems firmly attached to the ribs or can't be moved around at all. In any case, it would be best to have your vet take a feel of it. They might be able to do a "fine needle aspirate" as an outpatient - where they gently put a needle into it and see if they pull out fluid or any identifiable cells....I wouldn't suspect any sort of infection since it's been there for 2 months at least and isn't bothering her at all.
it is on her body behind the shoulder and it is slightly squishy and moves very little
It is possible that this is a benign lesion like a cyst or fatty tumor but there is some concern because this is a location that can be associated with a type of tumor called a fibrosarcoma. The fact that is is getting larger concerns me also. If is it a little squishy it might have fluid in it which would be a good sign but I still recommend that it be seen, palpated and have an aspiration done at your local veterinary hospital.
Ok, thank you so much.