Swelling in the back of the knee, hurts a bit. it that a cyst?

Probably: Swelling and pain behind the knee is most likley a baker's cyst. This is a fluid filled herniation off the back of the knee joint, usually associated with a torn meniscus (torn cartilage). It is usually worse with activity and relieved with rest. Treatment is usually nonoperative. If it bothers you an orthopedic surgeon can aspirate and inject it for temmporary relief.