What is causing this severe knee pain?

Never do surgery unless its necessary. You might want to check your doctor and get an MRI or x-ray and check too see if you might have partially tore or tore any ligaments in your knee. I've experienced this before from my jumper's knee and you shouldn't participate in sports that stresses your knee to much like volleyball or basketball. I think you should rest for now and do leg strengthening exercises ( since your leg muscles might be too weak so it strains your knee) like body squats, leg lifts, and lunges.