what can be my problem

Years ago i fell and went to emergency room the diagnosis was knee sprain and damage to the ligaments and was told that I would always have problems with it.And over the years I've have had minor swelling and elevating and ice pack would work fine,but recently I went on a outing where I walked for a mile or so up and down big boulders and the next day my knee was very swollen, well after a week of elevating wrapped and ice packs the swelling went down,now its got to where it feels like my knee locks up I cant walk and I get this horrible pain that radiates from my foot to my back of my upper thigh and if I bend it its just fine no pain but if I try to lay my leg straight it wont do it and the pains horrible,and if I move my knee back and forth the pain goes away and then i can stand straight and walk.Does anyone know what is happening and how do i get it where i can walk normal without it acting up