I Have A Thumb Sized Lump At The Top Of

I have a thumb sized lump at the top of my bum crack (sorry didn't know how to describe it). It is unbearably painful and I struggle to walk, skit and sleep because of it. I've had it for 4/5 days and it hurts more and more each day? At first I thought it was a boil but there is no sort of head and it is more painful than a boil. Any suggestions what it could be?