Help! My knee hurts so much now that I have gotten my full leg cast off, it only hurts when I try to bend my knee. Please help!

Physical therapy. After use of a long leg cast, the knee tends to be very stiff with discomfort during flexion (bending) being typical. Physical therapy, or at least home rehab, is often necessary to regain knee range of motion, regain flexibility and improve strength and function. This knee discomfort tends to fade away over a couple of weeks as you regain your motion.
Call your doctor. Why was your knee in a cast to begin with?