I have a similar thing, it's sorta like an ingrown hair that comes to a head often but in the exact same place every time, just on the outside left part of my nipple. During my last physical my doctor told me that it is just an enlarged pour that keeps plugging up. I guess that type of skin on your body never really heals like the normal skin. If you get a deep blackhead or something similar that takes up a lot of space, once you get it out it leave an enlarged pore.
Don't know if this is the same thing or not, but it sounds similar so I'm just passing down the "reasoning" my doc gave me.
Jdabs