My cat has a bump on her lower lip.. what is it?

My cat is an indoor Bombay mix. every once in awhile she gets a bump on her lower lip. It isn't colored but it just looks like the skin is raised. I don't think that it's painful because she lets me touch it. The bump is kinda sorta firm... it comes up almost over night but goes away after about a week. I'm not sure what it is, if its serious, or if i should take her to the vet or not.. please help!