While cleaning my bike after a ride yesterday, I noticed that ALL of the REAR SPROCKET BOLTS WERE LOOSE. Not one, not two, but all of them. They are the self locking type so they didn't come all the way off, but this seems awfully strange. They are supposed to be self locking, not self loosening. The bike has about 1300mi and it is the original sprocket and it hasn't ever been taken off. What other bolts should I keep in check so that it doesn't fall off on my next ride?