topflightpro , the bike was the cause of my back problems. Being hunched over and pushing stress across my lower back made it unhappy.
Training 100% on bikes (road, track, and spin) for a season confirmed this for me. That season my watt numbers where highest ever...and so was my back pain. I was straight debilitated. There were days where I'd stay home from work it'd be so bad. Then one day I went and did some squats at the gym because I couldn't ride. Guess what? The pain went away almost immediately.
Maybe it's my gut. But, whenever I have back pain, squatting with proper form and even light weights makes me feel 100% better.
I know this may be contrary to popular belief, but squatting may fix your back due to strengthening muscle imbalances.
Squats are amazing for rehabilitating injuries. When I tore my hamstring a couple of years ago being too aggressive with single leg press (pushing hard when the leg was fatigued), following the Bill Start rehab program I started squatting just a few days later, and was back to 100% sooner than I ever expected.