What is this?

I have noticed I've had this on my neck for a year now. My scalp is completely fine on stop but darkens the more down you get to my neck and it's only in the middle, the sides of my lower scalp are normal. I've had strange lump/bumps in the back of my neck and my scalp but the doctors did nothing to see what they were. It was painful at first but now they're hard and painless, right where the brown start on my scalp there's a hard lump in my scalp. I don't know what's causing the discoloration