What is wrong with my triceps muscle? The middle is ok but ends are aching.

Tendonitis. When people have problems with their muscles, the trouble spots are usually where the muscle turns into tendon and inserts on the bone. The triceps originates from the shoulder and the humerus bone and inserts on the ulnar bone in the elbow. I suspect that you may have strained the muscle at those two points. Sometimes, problems with the shoulder itself can cause pain in the triceps. See a doc.