s no reason to specifically believe that it was caused by fighting with another
cat, that I know of, unless the vet recognized a
cat scratch
at the
site. Is there any sign of anything like that? Does the lump feel hard or soft? Can you move it around at all (if you see what I mean) or does it stay put?
If it is an infection, she should have antibiotics prescribed and most likely, an antibiotic injection.
It could also be a bug bite, I suppose, in which case she might require antihistamines.