I have one inch diameter lump behind knee just above calf on inner side of knee, I've been stretching calf muscle last 3 days pain behind knee?

Bakers cyst. This is most likely a baker's cyst. This is a pouch of joint fluid collecting behind the knee. It can occur with most types of arthritis and with repetitive squatting. The most common cause is osteoarthritis. There are other more dangerous causes of a lump in this area. You should see your doctor to get a definite diagnosis. If you have calf pain and leg swelling go directly to the er.