Why is there a painfull lump at the top of my crack?

okay well I just noticed it last week, there is a pain full lump at the very top of my crack. Its rather embarring it seems like its getting bigger because at first it didn't hurt that much but now when I lye down on my back or sit at a certain angle it hurts really bad. I currently go to a university and I'm not sure if they can provide any treatment. what is the cause of this and why did it happen?