I have also had this problem from time to time. It seems to me that one of two things has to be happening:
1. Your face is getting in the way of the string.
2. Your string is getting deflected into your face.
If #1 is happening, you can try rotating your head more toward the target, holding your anchor more lightly against your face, or keeping your head in more of an upright position. Happily, these are all good things for your form too. You can also try changing from a brass string nock to a low profile tied on nock.
If #2 is happening, you can make sure you are relaxing your fingers rather than trying to open them to release the string. Look at the video Jim Castro posted in response to the "form" topic in this forum. It has a good explanation of this. Or, you can try a different tab or glove if the one you are currently using is scraping against your face when the shot is released. Some people have changed from a tab to a glove to avoid this problem.
Once this starts happening, any little brush of your fingers or the string against your face will be noticeable, so you may have to hold the string way away from your face or lay off shooting for a few days to give your skin a chance to recover. A certain amount of hand/string/face contact is probably unavoidable and goes unnoticed in normal shooting. It would be a shame to stumble on to the cure and not know it.