My lower right leg hurts, right behind my knee all the way to the top of my heel. Whats the reason?

Bakers Cyst. You may have a bakers cyst behind your knee that is causing pressure on the nerve and blood supply. You should have it checked by an orthopedic surgeon.
See the doctor ASAP. There are many possible causes, ranging from muscle strain to a blood clot in the leg. Please see your doctor for diagnosis and treatment.