Thanks for all the replies. Actually the pain came from leaning over and running along with my son as he was learning to ride his bike without training wheels. I did this for like three hours one day, until he got it. He kept pushing me to keep going, or else I would've stopped, hehe. I haven't tried the new setup, but I've got short legs, and was running a 175mm crank. I inherited the bike with those cranks on, and realized that they're too long, so I just changed it up. I haven't ridden the bike with the new setup yet, so I dunno how it'll feel. I'm just wondering if riding is going to make my back worse. I guess I'll just take it slow and ride. If it gets worse, I'll try doing some situps and changing my seat height and see if that helps.