Mole On Shoulder. Feels Like A Boil. Purple In Color. Worry Much?

hi i am a bit worried. i noticed about a month and half ago what i fort that looked like a mole on my shoulder , but a small lump as appeared underneath it and it really hurts, its not getting any bigger but the colour keeps changing from brown to like a purple colour, it feel like a boil but its not going, it feels like its getting worst because i cant put any preasure on it, i am really worried now because all my shoulder and going on to my back is really aching. i am booking appointment at the doctors tomorrow as i feel i have left it to late now because i dont no what it is. please could help me if you no anything?