Myself and 3 others set out to descend Monkeyface. I had been ill in the week leading up to the canyon, but was determined to go through because a friend was in town for the weekend. This was mistake number one. The canyon was in easy mode and I had descended the canyon a dozen times over the last 3 years. I was the last person to go down rap 7/8. Canyon was in easy mode, flow was relatively low. Having just been through the canyon a few weeks before, we chose not to re-rig at the chockstone since the new bolts were placed at the top. I deployed the rope bag and reached for the rope and am not entirely sure what happened, but my best guess is somehow I grabbed the non-blocked side of the rope. I stepped off the ledge and was in a sudden free fall and landed with my left ankle jammed into the chockstone. I have a memory of not removing rope from my device, just simply rigging the bolts above the chockstone and continuing down to the group. A group went through the week after and the original anchor was in good shape, so there was no failure there. Another possibility is a mistake in setting up rappel device, but it's hard for me to believe that any of those things actually happened. I continued to rapel/crawl my way to the bottom of the canyon. SAR was contacted and I was lifted to Loma Linda hospital and released later that night. Lessons learned, if you are not feeling well, it's not worth it no matter how much it hurts to back out of a canyoneering trip. Also, check, double and triple check. I may have become too complacent in the canyons. I safety checked everyone, but myself. Also, pull away the rope from the rapide. My eyes may have played a trick on me if I indeed went off the wrong side of the rope.