That sounds like a strain of some sort. If you slipped a disc or herniated something you wouldn't be able to move at all without massive amounts of pain. Of course, it is much different reading a description than seeing it, but I'd put my money on a muscle pull or strain.
I'm a kinesiologist by the way, so I'm not just making this up as I go. I'd recommend seeing a physio in your local area though. It's much easier to diagnose and help in person than on a message board. lol
Best of luck with it.