Did it ever occur to you that a tight hamstrings muscle can cause you back pain? Hamstrings is the muscle just under your butt, behind your thighs and extends all way down to the back of the knee. When hamstrings muscle are tight, they shorten, hence pull down on the pelvis and up to the lumbar spine which causes strain or an uncomfortable lower back.
Try lying down flat on your back and have someone slowly raise one of your foot. Your foot must be completely relaxed and make sure you dont help lifting your foot. On normal circumstances you should be able to raise your foot on a 90 degree angle (ie one leg on floor the other straight up facing ceiling). But, if you feel tightness before your foot reaches 90 degrees, i would advise you to stretch before it gets worse. Worse here meaning a lower back pain. People who sits alot (in the office, driving a car) have greater possibilities of a tight hamstrings.
Once you have identify you have tight hamstrings, do a warm up like a brisk walk or just walk around for 5-10 minutes and you can start stretching. By warming up, you get a lot of blood flowing through your muscles. When stretching, hold for a good 10 seconds and repeat on other foot. Remember not to bounce when you stretch.
People with back pain will find it beneficial if they stretch on a regular basis. It doesnt mean if you dont have a back pain you need not stretch. I would say, precaution is better than cure. Include hamstring stretch in your everyday regime. It will keep your lower back and pelvis balanced.
Some suggestion on the Hamstring Stretch
You can do it lying down. Lie on your back and support the thigh behind the knee with your hand or you can also use a towel. Slowly pull towel or straighten knee until a stretch is felt at the back of the thigh and get your foot to face the ceiling. Hold on position up to 10 seconds and alternate with other foot.
If you find this difficult, you can also try lying down on your back but your butt against the wall. Keep one foot on the floor and the other against the wall pushing it up to a 90 degrees angle. Hold position for 10 seconds and alternate with other foot.
Sitting hamstring stretch can vary the degree of the stretch depending on the placement of the foot.
Sit on the edge of a chair. Straighten one foot infront and place only the heel on the floor, toes facing upwards. Sit up straight, try pushing your belly button towards the thigh without leaning forward too much. You will then feel a stretch, hold for 10 seconds and alternate foot.