I've had back problems, including surgery, for a long time. Lower back pain is just a normal part of my life. The suggestions about developing core strength are spot on. Stretching too. Try this before your next ride. Lay on the floor on your back. Pull your left leg up to your chest and hold. Straighten it out and pull your right leg up and hold. Straighten it out and then pull both legs up and hold. Straighten them out and then remain on your back but bend your legs so your feet are flat on the floor. Keeping your shoulders flat rotated your hips as far as you can to the left and hold. Do it slowly. Repeat to the right. Switch to your stomach on the floor. Keeping your pelvisagainst the floor do a pushup arching your back and hold. Go ride. Repeat stretches when you return.