Generally this can be solved by turning your head more in the direction of the target. Your shoulders should be perpendicular to the target, as shown in Terry's clock elsewhere on this forum, and your head should be rotated at least far enough toward facing the target that the string doesn't hit your nose. Your head should be in an upright position; if your head starts drooping forward, it can also cause the problem. If you are also getting left hits at the same time the string is hitting your nose, it could be that you are releasing the arrow before you come to full draw, so that the arrow is pointed slightly to the left rather than straight at the target. Since the arrow is pointing slightly to the left, the string also is travelling in that direction, which could cause it to hit your nose.
I have also had shooting gloves that seem to direct the string into my nose, regardless of how I shoot, and of course I have gotten rid of those. I assume this isn't your problem since it has only happened in the last few days.
The string should travel very close to your nose, and it doesn't take much deflection to make it skim your nose rather than missing it. Some very good shooters have been known to wear a patch of tape on their noses to avoid this problem, but it would be better to avoid it entirely, if you can.