About 3 days ago I woke up and turn on my side and notice pain at the top of my buttcrack (sorry for the language). Here are some points...

1. Its about an inch below my tailbone, right were the crack begins

2. It only hurts when it is in contact, even more painful if I sit on with pressure but okay when I sit in normal position (pressure on thighs). Hurts when my buttcheek spreads e.g going to toliet or when I am actully pressing it when my finger. Doesn't really hurt when I walk or lay on my side (or on my back for that matter since most of the pressure is on my back). 

3. Its about the size of a coin.

4. The pain is under the skin, you can't tell its there by looking at it.

I have no idea what this is and I'm getting a bit worried. I was thinking it might be just a large pimple under my skin but not sure.

Do you have any ideas?