Lumpy swollen m in left help!?

im 16 years old ad ive had this for about a year now. its very hard and feels like theres is a bunch of lumps on this m that is about 2 inches in diameter arould the edge of my and is movable under my skin. it has stayed hard and swollen for about a straight month now and it's very very sore to the touch and is causing me pain that goes down my arm, what could this be?!!