I Have A Pimple Under My Arm That Got Big

I have a pimple under my arm that got big over the weekend. It's gown down but now there's a slight pain in my arm when I lift it above my head, almost the same feeling if I did a lot of heavy curls. Really Startin to worry. I have acne in my public and buttocks region and a Dermatologist said it was just bacteria and I have really fine hair. Im 29 a smoker and drink regularly. Just worried it could be more than acne since my arm is starting to hurt.