ORIGINAL: ijimmy
Do you use a peepsite ?
does it seem low on the string?
are you adjusting your eye down after you draw ?
If so you may need to raise your peep site .
Your nock very well may have sliped , as rybo said
Good way to find out is to close your eyes, draw your bow and anchor. If you have an kisser take it off for this as you want to use only your hand. Then once you are settled, open your eye. You should be looking straight through the peep. If not adjust it. Then try to re-sight it in.
Good luck.