First of all, you can experience excruciating back pain without having any earthly idea what may have caused it. Sometimes, just moving the wrong way can cause it...especially if you do not regularly exercise or stretch.
Get to your primary care physician. He or she should do a few simple tests such as have you lie flat on your back, tell you to relax your left (or right) leg,and then the dr. will lift it straight up. This is to see your range of motion and if there is any pain in your back when this is done.
If you have not "injured" your back from anything obvious, you may be having muscle spasms, or your hamstrings and back muscles could be very tight,and this causes lots of pain.
The first thing the dr. will probably do is to give you muscle relaxers to help relax your muscles, because I'm sure you're probably tensing up when moving because you anticipate that it will hurt. Possibly the dr. may give you some pain meds. too for when the pain gets real bad. They usually have you take the meds./relaxers for 1-2 weeks.
The dr. may also send you for an MRI to see if you could possibly have a bulging disk and/or x-rays to see if anything's wrong.
If nothing major is found, physical therapy can do wonders...after the bad pain is gone, exercise does help.
Nowadays, most doctors prescribe meds., stretching, exercise, and physical therapy instead of surgery for back problems...be very wary if surgery is even mentioned especially in the beginning.
Believe me, I understand what you're going thru...had the same sort of thing happen to me 3 years ago except with my lower back. My back muscles are weak, and my dr. told me I'll have to do stretches twice a day for the rest of my life if I want to avoid more problems. It's a small price to pay, and it really helps. Please let us all know what happens, and good luck to you.