Should I go and see an orthopedic doctor if I can't bend my knee for 5 minutes? I can't sit without my knee hurting

Yes. You may have patellar pain or a related condition. You could start by seeing a non-surgical sports medicine specialist.
No. . No. Five minutes is a short time. The real question is does your knee hurt when you bend it or does it not bend because it is "locked". If the knee is "locked" that means it can't bend because something is "caught" between the thigh bone and the shin bone which prevents the smooth flexing and extending motion. This may be a loose piece of bone or cartilage or a tear of one of the meniscus cartilages. Wait, relax, hot packs and massage and the knee may bend as it "unlocks". Repeated locking is cause for an orthopedic visit. If your knee hurts when you bend it, the causes are multiple and could be muscle or tendon or ligament or nerve that is being irritated when the knee moves. An irritation in the low back may also cause pain to be "referred" to the knee. Observe the timing the pain, what makes it better, what makes it worse and if any associated signs such as warmth or swelling are present. Then go to your family doctor or orthopedic surgeon arms with information that can help define the reason for knee pain when bending.