What does it mean if my knee feels like its a pinching pain every time I sit down or run?

Time to see MD. Pinching could be many things--at 19 you should probably be able to be active without pain. Could be you just need some rehab/physical therapy for something such as patellofemoral pain syndrome, but mechanical symptom of pinching might be torn meniscus, cartilage injury, fat pad impingement, plica, or other causes... See a sports medicine/knee specialist and get it checked out.