Tendonitis is inflammation, irritation, and swelling of a tendon, which is the fibrous structure that joins muscle to bone. In many cases, tendinosis (tendon degeneration) is also present. Tendonitis is also forever lasting, it sticks with you but you can keep the irritation and inflammation down. 

Now imagine all those things but on the main tendon in your shoulders causing every movement and lifting of arm to be irritating. I can't write for very long before it hurts. That's why I sometimes get to type essays that are meant to be written.