I think it's normal to have some pain - not _sharp_, awful pain, but _some_ pain - between the shoulders when starting on a new bike, or at the beginning of every season. I expect this pain to last for 300 km or so.

I know my bike is pretty well fitted in general but I hadn't ridden for more than 50km for more than a month and yesterday I went for a longer ride... ouch! I was hurting everywhere, and, yes, especially in the back. It seems like it's one of the areas of our body that is working a lot during cycling and perhaps not during other kinds of exercice we do.

So I'd tolerate it and stretch meticulously after every ride for another hundred miles, max. If there's no change, or if you start hurting somewhere else (elbows...), ask for adjustments. It's always a good idea to get adjustments after 500 km on a new bike anyway.

Enjoy!