I've found yoga to be REALLY helpful for upper back/hamstring/hurt-in-weird-places pain. You do have to be careful not to stretch beyond your limits like debbie says. Stretch just until the muscles are tight and hold it there... don't push beyond that with an injury! Once the tightness fades, you can go a little farther, but not too much more.

I usually use a yoga video, but there's this book, Hatha Yoga Illustrated, that lists a million poses and their modifications. The modifications are the ones I'm interested in! They show you how to make the pose easier so the less than flexible can do it.

For that area... cobra, bridge pose, down-ward facing dog... there's one... I can't remember it's name, but you put one foot forward, toe facing forward, the other foot back, toes facing forward. You face in the direction of your toes, put your arms behind your back, clasping elbows with your hands (like you're hugging yourself, but in the back) and then you fold forward over the forward facing leg. Don't lock your knees (you'll hurt them), but keep the legs straight... and stop once the muscles get tight.

Anyway... good luck! Sorry you're sore No fun.