Knee problem

OKay, I will try explain this the best I can...

Since I can remember I have had a problem with my right knee. To give a little bit of background, when I was born the doctor who delivered me twisted my right leg (so that leg points outward a little bit). Whenever I sit down, usually on my knees, or in a position that causes my legs to be bent more than a 90 degree angle my knee "locks" up in that bent position. Usually I can catch it before I stand up and "unlock" it, but last night it happened as I was getting up from a very low seat and since I was already committed to standing straight I had no time to "unlock" it, thus causing a few cracking sounds (like someone cracking a knuckle) followed by a pop sound. I knew today my leg would be sore...very! I don't think there is anything I can do about it but be very careful, but I was wondering if anyone knows what causes this.