Well, my best guess would be that it is an abscess. This is a sort of pocket infection that forms under the skin when some kind of wound, even a scratch, closes up too quickly and bacteria gets trapped. Cats get them a lot - it's very common.
There'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. Or, if there is no sign of a wound or scratch, it could be a cyst or growth, in which case you would need to have a needle biopsy done to determine it's cause.
Whatever is causing it, it is normal enough for her to lick it. Licking it means she's aware of it, that it hurts or at least feels funny or wrong. It's just instinct, they do it to keep it clean, as you say, and just because it's comforting. It can cause fur loss, though. If it is an infection, when it drains that will cause more fur loss, probably, and you will see one or more drainage sires - kind of like holes in the skin.
Getting a second opinion is always a good idea if you have any doubt at all about your cat's treatment. I recommend it in many cases, so I think you are doing the right thing. Something is causing that lump, whether it is an infection, or something else and you should know what that is.
I agree with Mrs_Savas in the sense that it does sound like an abscess. Hydrogen peroxide can be applied to the surrounding area but since you're taking her to another vet then I gues they'll give you some antibiotics to treat that lump or infection. You're doing the right thing in seeking a second opinion. It is good to question a vet's ability to perform, especially these days, when some of them can't seem to stop thinking of how much they'll charge you instead of how will they cure the animal.
Thank you so much for taking the time to respond, I greatly appreciate it. To answer some of your questions,
There is no sign of a scratch
The lump can be moved around it feels on the soft side.
I called the vet again today and she made me feel like I was crazy, so I am anxiusly awaiting the new vets opinion.
Again Thanks for your advice.
Thank you for your response. We clean it several times a day with peroxide and
antibotic ointment. Again thank you for your help!