I throw the same, and had the exact same issue. Went to see a doctor. I had strained my MCL. I now don't have this issue. Two things I did to fix it:
I got a knee brace. It cost me $30 at the drug store. This force me to not rotate my knee as much. It took about half a round to get used to, but actually increased my power quite a bit. After a while, it also helped me build better technique- even without the brace, I don't put nearly as much strain on my knee.
I also went to one session of physio and got some exercises to do. These should not be done if your knee is sore: let it heal first. But they will help it not get sore in the future. The main two are:
Stand behind a chair. Slowly stand up on the toes of your affected leg. Hold yourself fully extended for a few seconds, slowly come down. Once you can do this good, do it without the chair. Once you can do that good, close your eyes to do it (closing your eyes takes more balance and therefore more strength. Do this 10 times, rest, do it 10 times again, rest, do it ten times again.
Stand up with your feet a little less than shoulder length apart. Take point your right toe in as far as is comfortable. Lift your foot an inch, rotate it outwards a bit, put it down. Keep on doing this until it is pointed as far out as is comfortable. It should take you ~10 lifts to get it pointing all the way out. Now repeat that in the opposite direction until your foot is pointed all the way in. Repeat a few times.