I have a large brown mole, that seems to have an infection underneath it, possible cyst or something more serious? (Photo)

Hello, I appreciate your assistance in this matter. I have had a large brown birthmark on my neck and though it has grown over the years it's always been uniform in color and size. Recently though it's has been painful to touch around the sides, seems at times to be red ringed, and raised from the skin, as if sitting on a mound. I took a photo or two and noticed two "whitheads" on either side of the base. A bit of gentle prodding (truly gentle) caused it to erupt a thin lumpy pus. Concerning?