Two years after root c, one of my crowns becomes pressure sensitive?

I have four crowns/caps in my mouth. The most recent was done a year or two ago after a root c. The first cap to go on this tooth came off while I was eating skittles (lol) and was replaced about eight-nine months ago. Today and yesterday, it is mysteriously pressure-sensitive, and by that I mean the area beneath it hurts when any pressure is put on it, particularly contact with the capped tooth directly above it when chewing. It doesn't seem to be moving at all, what could be the cause of this?