Miner was switching out screens in Deister screen plant. He was moving a 50lb screen up a 12% incline when he stumbled, rushed to put the screen down so he wouldn't fall on it, and felt a ""pinch"" on the middle/rt. side of his back, although there was no pain at that time. Accident was reported to his supervisor 2 hrs later, after stabbing pain developed while in wheel loader.