First fall off my horse question?

I had my second riding lesson on my horse yesterday. He is stiff on his left rein. I am novice because I had a huge confidence knock a while back whilst I had this horse on loan and I basically needed to be taught to ride again. This horse gave me so much confidence! Even though he is very cheeky. So, what we were doing is trotting down once side of the menage then catering up the other side. He used to be extremely overweight but now that he has lost most of it he is becoming very strong. He would be a bit cheeky and try to canter all the way around the school. I managed to make him trot on the left side though. When we changed the rein he trots around on the rein perfectly but is always on the wrong lead in canter. I managed to get him into the right lead and we had a great canter on that rein until it got to where we trot. He bolted around the menage and then he turned a really sharp corner and then just stopped and threw his shoulder to the side. I landed on my head and shoulder quiet hard. I really don't understand though? My shoulder doesn't hurt one tiny bit! My neck it a bit stiff today though. I have a HUGE bruise on my knee and my ankle hurts SO much I can't put any weight on it! I have hurt my ankle before but never so much I can't walk on it. But how did it happen? I landed on my head and shoulder, not my ankle? It's kinda weird. And what should I do about my ankle. If it help he is a 15hh cob so It wasn't a very big fall. This was only my second fall ever and my first fall off him. But I DID get back on and had a nice trot around, not very well though because my ankle hurt. I know it is a stupid question, but how did I hurt my ankle?