This is to help out anyone who may be afraid they have cancer or something equally as horrifying in their mouth. I noticed I had a small pea sized lump inside of my lower right lip about a month ago. Instantly, thoughts of cancer and sexually transmitted diseases shot through my head sending me into an absolute hypochondriadic panic. I eventually went to a doctor, who drained the lump (a clear, thick, mucously and salivary fluid came out of it) and sent it off to be cultured. I then was told it was seeminly normal and was not caused by infection or anything else. I dismissed it as being insignificant until it returned within a week, and larger. After it became a nuisance to my speech, I drained it myself in a sanitary manner, and did this two to three times a week. Just this past week I finally figured out what it was due to the clinical rotations I do at a children's hospital. I was so relieved to know that is merely caused by some sort of trauma (though not always) to the salivary glands in my lip. The lump is caused by the mucin draining into the connective tissues in the lip, and it is not technically a cyst because it lacks epithelial lining. Mucoceles are typically painless (to my knowledge) and don't contain blood or colored pus (this may be indicative of something that requires more suspicion). It is a simple surgical procedure to remove it (I'm having mine taken out at the same time as my wisdom teeth) and they also remove surrounding minor salivary glands to eliminate chance of reoccurance. They recommend sending it off for a histopathology exam to rule out any chances of it possibly being a malignant tumor (there are some rare instances of this).
So...hopefully this provides some sort of reassurance for those who are fretting about their lip mutating.
-Cristina-
Lump on lower inside lip? 
