Sprained my knee 4 mos ago and recently started getting sharp spasms, what could be causing this?

Many things...: Your knee sprain may have resulted in a number of cartilage, ligament, tendon, or meniscal injuries that are now causing locking or "spasms." i highly recommend you see an orthopedic surgeon to further evaluate your injury with a thorough history and physical exam. Imaging studies may also be necessary.

Knee sprain: Several possibilities: most commonly, torn cartilage, meniscus tear, ligament injury or sprain. You should see an orthopedic surgeon if you still have problems 4 months after a "sprain".