Why do I keep twisting/turning over my ankle?

Ok the first time was when I had just been on quite a long walk with my family, and we were coming down these steps and I turned over my ankle on the edge of a step and it swelled up like mad. Its happened overall 4 times in about 1 1/2 months. Once on that walk, once jumping around in sport, once while running and the last just walking out of a shop. The last two have been in the space of a week (Tuesday and Saturday). Every time it has been on my right ankle. Almost two years ago I broke my left leg (the one im not twisting) but I don't think it has anything to do with that because its all healed perfectly (even though it was a bad fracture) and i've had no other problems. My mum said my ankle might be weak but I don't think it's that weak because I do a lot of walking around my school every day and I do 3 hours of tennis on Monday, 2 hours of fencing (sword fighting) on Tuesday, 3 hours rock climbing Wednesday, 1 hour athletics Thursday and 1 hour tennis Saturday; as well as cycling, dance and a few other hobbies. I'm not twisting it because i'm doing too much sport either so please don't bother putting that (I know this for a fact). Can you please tell me why I keep on twisting my ankle and also what I can do to stop it twisting? xxx