I sympathize. It happens to me from time to time, even though I've shot 3 under for years. The thing is, even under the best of circumstances, the string is never very far from your nose. A tiny rotation of your head can move your nose into or out of the path of the string. Brass nocks make it worse, at least they hurt more than tied on nocks if they hit your nose or lip. Pressing hard into your anchor makes it more likely you will hit your face with the string, and is also likely to deflect the arrow, so keep a light contact with your face. If you try to release the string, rather than just letting your fingers relax, it can roll the string into your face. Lot of stuff to try and figure out.