The vet is going to be the best person to figure out what's going on with your kitty, although one possibility is juvenile-onset stomatitis: http://www.sniksnak.com/cathealth/stomatitis.html
Good luck with the vet visit, and let us know what they say.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler