What could this be or mean?

under my left armpit i have a small lump. Its only small, like a large kind of spot. Except its not a spot, its feels like a ball under my skin, and its really painful. Its not from shaving or anything. Its making my whole arm ache, especially the upper arm and it hurts when i raise it even a little, and my arm feels extrordinarily heavy. I have no idea what it is, but its not from shaving so if anyone knows what it could be, please let me know:)