2 Years Ago I Injured My Knee But Never Went

2 years ago I injured my knee but never went to a doctor. I stretched it while I landed after catching a football. I have been very active since and sometimes I feel pain in the back of my left knee on the left side. if I sit too long it also feels stiff. My left knee is noticeably weaker than my right one. Any idea what could be the cause of this? I am confused because when I play sports I do not feel like I need to hold back at all. Thanks