My first thought was scrunching shoulders, too... or locked elbows, which will transfer road chatter up your arms and into your neck... or not keeping your chin tucked in enough. Also, were you consciously changing hand positions regularly? I've ridden my road bike at least a couple of times/week for most of the winter, but had a 3 - 4 week hiatus when we were traveling. After that break I could feel it in my shoulders on my first ride out. During the next ride I made sure to move my hands around more, and a couple of times to sit up tall and do some on-bike stretching mid-ride and it seemed to help. It took a couple of rides to feel back to normal, though.