Suggest Treatment For Pain While Lifting Arm

I am 29, 1.57 metres, weigh 51 KG and have no medical histrory. I was treated for a boil under my arm about two weeks ago. The boil has gone down, however i think my tendon is caught in it as it is pulling and feels like a string. I am unable to lift my arm up completely as it is quite painful. The pain travels from my armpit to my elbow - what should I do?