i dont think the stretching helps it, it may only 'feel good' but it doesnt help
the lower back muscles seem so unconquerable compared to other body parts that it pisses me off that there is no way around the pain except to not bother it 
If you really want to know what is going on in your lower back, I can tell you. As I say earlier, it is all about the muscle balance. I bet you prefer sit ups instead of crunch, you don't do hyper extensions at all, and instead of that, you continue that stupidity which you think is "working out". The balance of the muscles in your middle body is important, because it effects to your posture. For example, if you do sit ups, but doesn't strengthen your lower back with same measures, you hip flexor muscles becomes stronger, thicker and same time shorter, and it make your pelvis tilt forward because your muscles in your lower back isn't equal in strength. When this happens, you will suffer lower back pain, meaning extended muscle soreness after any work and pain in your spine because the wrong posture. With lower back you need at the same time some hi rep/low resistance exercise, and some low rep and high resistance exercise to maintain your posture and health of your back. High rep hyper extensions, stiff legged dead lifts or good mornings for endurance, and dead lifts for strength. For abs, just crunch, no sit ups at all. Crunch on the floor, crunch with the rope, crunch in the machine. Sit ups are worst piece of shit for exercise your abs and in same time it is the first exercise you ever learn. That's why everyone is doing it, no because it is good for you, but because they learn it in their childhood.