Problem Under Mole on Neck

So last night I had felt that a mole, which is located on the back of my neck a little up into the hairline, was starting to get really swollen underneath like it has a pimple growing and filling with puss. This has happened once before and went away after a while, but I don't remember how long, but I do remember I got headaches and neck pains from it because of how painful it is. I have no squeezed it just to make sure I didn't hurt myself internally and I would like to know asap how to treat this and what it could be. Thank you.