Personally I wouldn't do any back strength training exercises until I had my lower back checked by a doctor or chiropractor. You don't want to risk injuring something that may be out of place or inflamed.
Stretching shouldn't hurt anything as long as you don't overextend or bounce.
Some good stretches for your back are toe touching, laying on your back and pulling your knees up to your chest with your hands under your knees, sitting on the floor with one leg stretched out in front of you and one bent and reach toward your toes (switch legs after about 10 seconds). Also, you can keep one leg straight, the other bent (crossed over the opposite leg) and twist to the opposite direction looking over your shoulder. For example, if your left leg is straight and your right leg is bent crossed over the left, your left arm would go on your right knee. Your right arm would be behind you to support you while you twist.
I am sorry if these are hard to visualize. I can show it 100% better than I can explain it in words!
Hope this helps!