Warm up before you stretch!
Studies have repeatedly shown that stretching first- before you have done 5-10 minutes of exercise that gets your heart rate up and the blood flowing through your muscles- can weaken the muscles, ligaments, and tendons, and actually increase the likelihood of injury. At a minimum, if you’re hurting, walk as briskly as you can manage for 5-10 minutes and move the major muscle groups that don’t make you hurt.
Hurting Back Stretch
The best simple stretch for your back is to lie on your back on a firm surface with your legs bent, then stretch a knee gently toward your chest. Use your arms to pull the leg toward your chest, then relax into the stretch for a full minute. Do one leg, then the other, then both together, relaxing into the stretch for a minute each time.
You can add dropping the knees (held together) to one side and then the other, again relaxing into the stretch for a minute.
Don’t overdo. Twice through the above sequence, twice a day (morning and evening) is about right for a hurting back.
It’s a great stretching routine, among many other benefits, but don’t try to do it from a book or a video. Take a class. Yoga postures are very powerful stretches, designed to be done with an Eastern sense of breathing, relaxing, and centeredness. If you do the postures with a Western, calisthenics-like sensibility, you can hurt yourself badly. Even a few degrees of change in the angle of the postures can create a wholly unintended stretch that can tear tissue. You need to have someone watch you closely at first until your body learns exactly how the correct position feels. Then you can use a video to pace you and remind you of the positions.