So I've had this lump on my left side towards my back. It's been there for quite a while (I'd guess three months or so? But I haven't really been keeping track of it) and at the start I thought it was just some sort of pimple, so I didn't pay much attention to it, especially considering it's location is not one I usually have reason to crane my neck around to view. It initially looked like a pimple, it was just ever so slightly darker than my skin colour, smooth and very small. But last week or so I noticed in the mirror it has a dark spot in the middle, like some freakish blackhead S:
Here's a photo I took !

Anyone have an idea of what it could be? D: