Why do I fart every time I kiss someone?

Ever since I can remember, when I kiss someone (someone I am in a relationship with, so.. on the lips of course), I fart. It is almost instantaneous. I don't understand why this is happening, but it just does. My lips meet theirs, and then one rips out. it is terribly embarrassing, and it has gotten to the point where no one wants to kiss me, because they think it is foul. It only seems to happen when I kiss someone on the lips, not ever on the cheek or anything away from the mouth. I just don't understand why this happens, and it eats me up. I want to have a relationship where I can kiss someone without grossing them out! Can anyone help me?