I Have Trouble With My Knees But I'm Not Sure

I have trouble with my knees but I'm not sure this is anything to do with that problem. I had a total knee replacement on the left leg in June. My right is not good but don't want to have it done yet. The problem is my right leg slipped forward today with my wet shoes. The pain was immediately very bad in the back of the knee. I now have this terrible pain in the back. I have no idea what it is but it is a concentrated pain on the right sight behind the knee. It's like a tendon or ligament or something. Can you help?