Yeah take it to the vet more than likely its whats called an rodent ulser its not from eating rodents or anything heres and article:
Eosinophilic Ulcer (Rodent Ulcer) In A Cat more than likely if it is the vet will put the cat to sleep and scrape it out, it does hurt them thats why they wont let you see it. If you let them go it will get bigger cats have lost parts of there mouth because of it. Look around for a cheap vet the procedure only cost me less then a hundred however the vet will more then likely try to put your cat on steroids first which is cheaper to see if it works, my cat was more suveer he had a swollen lip then started scratching raw holes in himself her is my article
http://www.askmehelpdesk.com/cats/sw...ck-186364.html swollen lip can also be an allergy so get it checked out good luck!