I Have Been Having Pain In My Arm Pit -

I have been having pain in my arm pit - left arm. It started a week ago as just a tenderness, touch made it hurt. It has progressed today to a sharp stabbing pain a little below the armpit on the inside of my arm. That part of my arm is very sensitive to touch. Hurts to brush the skin. Stabbing pain is intermittent. Is this something I should worry about? I don't have any other symptoms.