I suddenly tonight got a pain in the crack of my bum i've had someone look at it and found that there's a dark red line with some open sores going up?

Needs evaluation: With this sort of painful rash - I would recommend showing it to a doctor. That way you can get an accurate diagnosis and start the right treatment for what you have. I wonder if there was any 'risk' you might have engaged in to cause this? The answer probably lies in the history. This needs treatment though because you don't want it getting any worse- particularly in a sensitive area like this.