Whats wrong with my knee?

I hurt it 2 years ago running and it never quite got back to 100% but I hurt it again last weekend skiing. I have pain when i walk coming from the center left part of the knee. I also have bad pain when i stand up after sitting with the knee at a 90 degree angle for over an hour. Also if im sitting in a chair and extend my leg straight out and rest my heel on a box and relax my leg i get extreme pain from the interior of the knee.