What Causes Painful Movements In Arm?

I am 53, diabetic, overweight (272), hbp, high chloresterol..I tried to pull my 4 year old grandson (a lightweight) up onto the couch next to me 10 days ago with just my left arm...and i felt something in my left arm - like maybe muscle tearing or ?...sometimes I can do light things and it does not bother me...but sometimes even holding a light item - loaf of bread, whatever, causes pain...it hurts mildly all the time, but not much ability to lift or carry anything...if i try to use left arm much at all it is very painful.....what could it be and what type doc should I see?