I have the same rifle and never had that problem. You might be getting some blow back through the nipple, and what your feeling is a small piece of the nipple it self. Wear shooting glasses.

I really am surprised that this is happening, with the bolt sliding forward to strike the nipple I would think anything blowing back would move upward instead of backward. Perhaps you have a scope.

Let us know if you discover what it is that is hitting you..... thanks.