I had had what i thought was a pimple under my lip line for about a month.. it is small but will just not go away. what could it be?

Mucocele possibly: The bump on the inside of your lip is probably a mucocele. It is caused when the duct to a minor salivary gland gets clogged or traumatized and the saliva backs up forming a fluid filled bump. They are most common on the lower lip but can occur elsewhere in the mouth as well. Small ones can resolve on their own. Large ones (as shown here) require surgical removal by and oral surgeon.

Mucocele : is a very common condition that happen when a salivary gland is injured or blocked. A mucocele usually is removed by a small oral surgery. The dentist may use a laser to remove the mucocele for faster healing and less complications..