My friend has a small (pin head size) flat blister on the very center of his bottom lip. He's totally freaked out, but has not yet gone to the doctor. He popped it at one point and it was full of blood, so I assumed he had just bit his lip and had a blood blister, but after 2-3 weeks, it still hasn't gone away (still looks like it did before he popped it, blood under top layer). The blister doesn't protrude very far, and this is the only one he's ever had.
He's worried about his girlfriend and doesnt want to give her something contagious.