Is it where your lower lip curls back over your teeth, and your teeth are digging in too hard? Do you notice it particularly bad when you play alot in the upper range of the horn? If so you could be biting too hard. Biting = Pinching and pinching is never a good thing. If you've been playing for 5 years it may be hard to un-learn that, but biting down hard can be detrimental to overall sound quality, especially in upper range and altissimo