As MaHaTa said, what you describe is hard to imagine. I'll throw out a possibility: If you're having a consistent 'slap' as it sounds from your description, and you don't have a raw or bloody spot, I wouldn't think it would be the string. I've yet to see a string slap mildly. I sometimes get a string slap to my lip, and if I don't correct my head position immediately, three or four shots later, I'll have a blister on my lip, if not a bloody lip. So, are you sure it's your string, and not, perhaps the cock feather on your arrow?? I could see that happening if you have an anchor point with the nock near the corner of your mouth? Some shooters actually use the cock feather against there face as another point of contact to check for a consistent anchor point.
Best I can do from your description: