This pain sounds very uncomfortable. I have to say that a pain in this area of the body is unlikely to be tendonitis. Tendonitis literally means inflammation of the tendon. Often we don't know exactly why it happens, but most of the time it occurs because of over use of a the muscle it is attached to. Most cases of tendonitis occur in longer tendons such as the wrist, shoulder, elbow, knee, and ankle. Your situation sounds more like a muscle spasm or muscle cramp. I wonder if placing a heating pad and stretching this area would help your situation. If your doctor
clears it, you may find that using low dose non-steroidal anti-inflammatory medications may help this pain. Finally, topical medications that increase blood flow to the area can reduce inflammation and make your muscles feel better.
I suggest that you schedule an appointment to see your primary care physician
. He or she will listen carefully to your symptoms and perform a thorough physical exam. If your doctor can make a diagnosis, then treatment can be initiated. If not, then your doctor may try a short course of a medication for the pain which may not return once you have it under control.