I would say that it was my broken arm when I was young, but I don't really remember how painful that was.
So I will say it was when I was in college and taking Tae Kwon Do. I fought in a round-robin full contact tournament. I was very sore at the end, but otherwise felt pretty good. When I woke up the next day, I could not sit up, and the left side of my torso was killing me. I could not lift a five pound weight with my left arm without excrutiating pain. It turned out I had twisted a rib. It took a couple of weeks to heal, during which I could not lie on my left side, and to get up from lying down I had to roll onto my stomach and push myself up (primarily with my right arm)