What Causes A Red Welt Between The Thigh And Tail Bone?

I am a Male 48 year old quadreplegic. I have a red lina approx a 1/4 " wide red welt under my skin starting on the side of my left thigh going up and around almost to my tail bone. I have been to the ER and te doctors office neither place could figure out what it was. It could not be a pressure sore because I sleep on my right side and the mark is on the left side which is up in the air and does not touch the bed and it deffinatly happened at night in bed.