I knew she was a great cat!
Anyway, it might go away on its own. It depends on what is causing it, and sometimes there is no way to know. I would feel more comfortable saying to ignore it if you post a close-up photo. I will see if I can find a picture, myself, so you can compare it to your cat.
Here is a client-information article from an excellent site called VeterinaryPartner.com http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=656
Here are some photos:
What a cute cat! Her lip does look like an eosinophilic granuloma. As it stated in the Veterinary Partners article, "Keep in mind eosinophilic granuloma represents an inflammatory reaction to proteins deemed foreign by the eosinophils." Given her young age and the fact that she is an indoor cat, without fleas, I would vote for a food allergy as the most likely triggering cause. Here is a link to another Vet Partners article that gives more information on food allergies: http://www.veterinarypartner.com/Content.plx?P=A&A=652&S=0&EVetID=0
In addition, go through her environment to see whether there is anything you might have overlooked.