This may apply or not ... if you are moving your jaw around alot while you play it will rub up against your lower lip area and cause chaffing. Lips trills and jumps found in the Haydn are the kinds of phrasing that are easiest to notice it on. Try playing and look in the mirror. If it's an issue PM me and I will shoot you a copy of something one of our pro instructors sent me.
Always best to have an instructor work you through these things