It is worth getting checked but it looks to me to be what is called a Rodent Ulcer or Eosinophilic Granuloma (sp). It isn't serious or life threatening and can be treated with steroids and antibiotics or can even be left alone and most cases with resolve on their own. Sometimes they will ulcerate in which case you want to be sure they are treated. They have found that with these Ulcers they not only get them on their lip but in the esophagus and digestive tract. Does she vomit for no reason or have loose bowels? If so it is a part of this syndrome. Usually the swelling is red and hard and looks really sore but the cats don't seem to be overly bothered by them.

Good luck. Both my Brianna and now Deirdre get them. Brianna has had them all her life and she is now going on 10. Deirdre just started getting the swellings on her lip a few months ago but has had some of the other symptoms for a couple of years. Deirdre also has trouble every winter. I think she has a seasonal disorder...*L*

Denyce