You mentioned falling onto your back, I fell on my back, and thought nothing of it, I was winded for a few minutes, but otherwise good. Went to doctors and he said i had cracked my ribs. It only hurts when i sneeze. But when i bang my knee on the stem, i go through almost the same thing, Loss of balance, sickness feeling, dizzy. After about 5 minutes ill be fine, i just think its my bodys way of dealing with things, I do tend to have a fairly high pain threshold. Which makes banging my knee painful. I also recently experienced this, getting out of the car. I banged my elbow, and i was in excruciating pain for about 5 minutes. Afterwards i was fine. It maybe just be todo with the extremeties. E.g. Arms and legs. Hope this helps you to feel, not alone in your pains! From Lee