Alright, so these ladies are smarter than I. So, this is not to be a trouble maker, this is what *I* did.
Spin the tires - do they spin freely?
Make sure there is no brake rub...
Make sure the tires are fully inflated...

All of what everyone else is saying is probably exactly right, but I'd go back to the basics and just make sure your new bike is mechanically smooth sailing and you aren't having a mechanical issue.
I've done these things, ridden 10 miles "uphill" only to find my brakes were rubbing!