Are you doing cardio on a treadmill? I once strained my lower back and had to see a chiropractor. He asked what I did for training and I mentioned that I did cardio on a treadmill. He said that it is the worst possible thing to do with lower back problems. I can't exactly remember what the reasoning was. Needless to say, I still do cardio on treadmill, but have not had any back problems since seeing him.
Try using some other cardio equipment and STRECTCH!!! Some of the best lower back stretches that helped me are:
1. Lay flat on your back and pull 1 knee to your chest at a time and hold for 15 seconds. Then switch. Do a few sets of these.
2. Again lay flat on your back. Rotate your hips and legs to one side of the body with the top leg stretching further out then the bottom leg. Your torso and arms should rotate opposite. Hold this position for 15 seconds and switch.
3. Your hamstrings can have a huge effect on your lower back. If they are injured, you may feel some of the pain in your lower back. For that, lay flat on your back and keep one leg on the ground and lock your other knee and raise your foot as high as you can. This is a killer if you are not too flexible. You can use a towel on your foot to help pull the stretch more or have someone push your leg higher for a stretch. Hold for 15 and switch.
You can search for any of these on youtube.com if you need a visual. These three have helped me immensely. And keep doing them even after the injury subsides.
Good luck!