Muscle/ Mcl or idk in my leg need help bad!!!!?

Since the pain in behind the knee you can eliminate the medial collateral ligament. Going up the right side of the left knee would be the medial hamstrings and if it is the right knee then it would be the lateral hamstrings. If you are stating that it the medical collateral ligament then the injury would have to be the left leg. That pain would be in the center of the knee. I just wish that you had been more specific to what you are feeling and which leg. What movements or actions are giving you problems?