I have pain in the back of my knee with some swelling. Should I go to the er?

Knee pain. This does not sound like an emergency. Call your doctor to be checked for the cause.
Not generally ED. Generally pain the back of the knee with a discrete lump which gets more prominent when you stand is known as a popliteal or baker's cyst. Benign but sometimes painful. Should see your regular doc at next available visit.