I have a pain on my right side going around the back of me to my bum. Can't sit down or lie down?  

Could be... This sort of pain could be caused by a lot of things including a pulled muscle, muscle spasms, a pinched nerve, a kidney stone, a kidney infection or any number of other things. If it's as bad as you're describing, you'd be wise to get it checked out.
See you Dr. Sound like you may have "sciatica" see your orthopaedic spine specialist and get examined along with an x-ray and mri.