why does my bum hurt?!

OK so this is kinda weird. And I don't really know where else to ask about it. So everytime I sit down, stand up, or change positions(pretty much anything) it hurts. It's not really my butt, but like the very bottom of my back as it meets my booty. And it's only in the center that it hurts. And it doesn't hurt when I press on it or touch it so I know it's not a bruise. And it doesn't look any different. Bit it hurts really bad and it's been terrible for the last week. Does anybody have any idea what it could be or what caused it?