I woke up with tender pain in the middle of my left arm by the bend. I did not do anything that would have bruised or strained it. What could it be?

Many choices. Not sure exactly where you are saying it hurts. But, epicondylitis can be a source of pain about either the medial (inside) elbow or lateral (outside) part of the elbow. It can be quite painful but is usually a self limiting condition.