keep spraining my knee?!

so about seven months ago i sprained my knee falling off a platform(we were visiting an old school playground) and i know this isn't advised but i had to walk about a mile back to my friends house because we did not have a ride. when i fell i heard a series of cracking noises(like when someone cracks their knuckles) when my knee twisted and then twisted back into place. after a couple weeks of resting and icing it everything seemed fine, my knee felt a little unstable and there was pain on the inner side of it but other than that it seemed much better. about two months later i was at work and turned to walk around something and my knee twisted the exact same way only this time i was able to sit and ice it right away and took medicine for the swelling. i went to the doctor for x-rays and everything was fine and he said it just seems like a sprain that should heal on its own, but i never got the chance to go for an MRI that he recommended. in gym today we were rock climbing and i didn't think i'd have a problem but off course with my luck(and being suspended 15 feet above the ground) it twisted AGAIN :( only this time it was only one cracking noise when my knee popped in and out and the pain was not as great. i iced it and ive been wearing my knee brace that i got the second time this happened and we're making my doctors appointment this week but my question is what exactly is wrong with my knee and will i need surgery? or will it heal on its own? if so how long? any details/tips/advice you could give me would be very much appreciated :) thank you