Hi there,
This is actually my first post. I'm returning to road cycling myself after 15 years off from a back injury, with cycling being the last activity I love that still gives me an issue in my back. I was seeking a good forum to share with other riders and discovered this forum and this thread. Hello.
It sounds like you may have a disc problem, which is something you should be very cautious and careful of if you do.
I have to do yoga for an hour a day to keep the back in a strong and limber state so my spine doesn't kink down there in the lumbar area. When it's bent and I am in an unnatural position atop a road bike, the rattling of the bumps in the road are like small bombs if I am not careful.
If you have insurance that allows it, I recommend you get an MRI if you continue to have pain when you ride. If it is not caught, it could end up being a daily problem and not just occurring on the bike.
Before I ride, I do a session of yoga exercises to get my back warmed up and limber. If I don't have an hour to exercise before, I simply cannot ride. Otherwise I'll be laying on ice packs for the next 3 days.
When I ride, I keep my seat tilted forward a bit, which forces my pelvis into a more accommodating state for the spine to rest. I may be losing some energy in this way, but I'll lose a lot more energy if my back goes out. I also stay conscious of my back's posture at all times. The moment I forget, my back will go out. i have also learned the importance of arm strength in cycling. basically I learned that all muscles need to be as strong as the leg muscles to really stay balanced and injury free when on the bike.
Just because the pain is in your back doesn't mean that is the source of the problem. In my case, My back went bad due to my weak hamstrings and knees--a result from sitting in office chairs too long in my life. The myofascial tissue that wraps all your muscles and organs together acts like a rubber band and can start injuring an area near it, making you think that's the area that really needs the work. Basically, you get 2 injuries for the price of one.
Look into Iyengar Yoga & Pilates classes in your area. They are both recuperative forms of exercise that are becoming widely accepted by people in physical therapy and medicine.
Hope this helps a bit.
-Court