It is an instability in your vertebrae. I have the same problem.
My wife is a physical therapist and she had me do a series of exercise to strengthen the muscles around the vertebrae. It takes some time so you may pinch it again until the area is strong enough to keep the vertebrae in line.
Posture can also cause it. I sit behind a desk so that is the source of my problem.
Basically, you lay flat on your stomach, head down for all of these:
1) place your hands straight ahead of you, palms down, about 4-5 inches off the ground and raise them as high as you can, hold, then lower them to the starting position. Do 4 sets of 8-12 reps. Do not use weights.
2) Same deal except your hands are straight out to your sides, palms down, raise them up, then back down, 4 sets to 8-12 reps.
3) Same deal except palms facing forward. 4 sets 8-12 reps.
A modified version you can work out is have your arms at 45 degrees (in between straight out and out from your sides), palms down, 4 sets 8-12 reps.
I do this on shoulder days and on my ab & lower back day (I do a normal 5 day workout but rather than take two days off I have a day where I focus on lower back, abs and mid back where I have the impingement).
I tried to find a good pic or vid on the net but I couldn't.
It is similar to the superman found here but you are raising both hands at the same time and you are not raising your feet. I keep my feet off of the ground, keep my abs tight while raising my arms.
http://beebleblog.com/2007/02/14/back-exercise-the-prone-superman/