Tendonitis is a tricky thing to diagnose---it can happen anywhere, elbow, knee, you name it. Don't wait---go see an orthopedic specialist and get some X-rays, even an MRI, and follow the doctor's instructions as to how to deal with it.
Let me tell you a story about what happened to one pitcher.
Eddie Lopat had won both games he had pitched in the 1951 World Series, but after the second game he suddenly couldn't lift his left arm. Ouch. He saw several doctors, but none of them could tell what was the matter with him, and so the start of the 1952 season saw him on the shelf---what is now called the disabled list---which he didn't like at all, because he wanted to PITCH, was what he wanted to do! Then he remembered an orthopedic surgeon in Chicago, whom he had known in his White Sox days, and he flew out to see him. The doctor examined him and then said, "Eddie, you have tendonitis in your left shoulder!" Ouch. Then, after chewing him out for not having taken care of it sooner, the doctor prescribed what was then a new and radical (though no longer used) treatment: a series of ten X-rays to that shoulder. Radical, yes---but it worked, the way it had worked for some other pitchers who had tried it for a similar problem. When Lopat returned to New York he was pitching better than ever; from then to the end of the 1954 season he racked up a 33-8 record and continued to beat his favorite patsies, the Cleveland Indians, to an unrecognizable pulp.
So---get it checked out by a specialist. Something can be done. :baseballpitcher: