My long term problem turned out to be just plain old muscle spasm. It happened every once in a while in the past four or five years or so. I remember the first time it happened I'd bent over to pick something up and my back was stuck like that, it was really bad cause I had a final the next day and couldn't continue reviewing. At first family thought I was faking it and trying to run away from studying, but when the realized that I really couldn't straighten my back they kinda freaked out