I noticed last night a painful lump underneath my left armpit. It feels like the size of a pea and is underneath the skin. I've noticed it has been "puss-ing" a white-ish color. The lump protrudes a bit onto the surface of the skin, where the puss is escaping from. When the area is touched or irritated (e.g., moving my arm, etc) - it hurts a bit.

Is this something I should have checked out by a doctor? I've never had anything like this before.

Thanks for any advice you can give.