I'm guessing you're having back pain when you're not on the bike, too? I'd work on it from that angle first. Bulging disks were probably not caused by poor bike fit, although that could have exacerbated it.

I've had disks repaired twice in my back, so I feel ya. Core strength, stretching like SW said, strong hams and quads...that's the insurance that keeps your back in good health. I'd work on the back issues off the bike first.

Karen