When i walk, i get a cramp in the back left side of my left knee. it hurts so bad that i'm not able to bend my knee for a while. what is possibly wron?

Knee Pain: With activity suggests a mechanical problem in or around the knee. Possibilities are tendonitis, arthritis, cartilage tear(medial meniscus), or ligament strain (medial collateral ligament). Vascular disease is less likely. If this has been long-standing, or is associated with knee swelling knee, or persists for more than a few days, see your physician or an orthopedist, for evaluation/exam/mri.