Where exactly does it hurt? You said lower back on the far left and right sides. But I'm still trying to figure out the spot. Wish we had a chart we could draw on . . .

I wondered about simple back pain or kidney pain, too. I get lower back pain when I run too hard or too long. Stretching out and warming up beforehand help a lot. Sometimes also, people doing cardio exercise get a "stitch" in their sides. That's usually up higher--like along the bottom of the lungs in front or on the sides. That happens when the abdominal oblique and diaphraghm-supporting muscles cramp up, and "stitch pain" will ease if you slow down your exercise pace, put your arms over your head to stretch those muscles, and breathe deeply a few times to take in more oxygen. Try that just in case that's what's happening. I know from experience that running's definitely hard on the back, and sometimes your posture while running can make that worse. The best way to strengthen your back is to strengthen your abdominal muscles, so try adding some stomach crunches to your routine and see if that helps ease the back pains, too.