What Causes Painful Weight Bearing In Leg?

I am having extreme pain in the back and outside of my knee. I am unable to put pressure on the knee without feeling like I am going to fall to the floor. I was in the E.R. on Monday and all they did was send me home in a knee brace on crutches and gave me pain medication. I am currently on 500 mg of lortab and still feel pain as if I am not on any pain medications. Should I return to the E.R.?