my back hurt for a few weeks when I first started, and I haven't had a problem since. I think it's just muscles getting more use than they're used too.

And I would second the idea of going to a professional about a bike fit. I went a few days ago, he put me on an adjustable bike, talked about ride position and it was AMAZING. Rather than adjust my current bike, he's building me a new one. But I've already noticed a difference since taking is tips on body position.