The naproxin is a muscle relaxer. It's likely you had a muscle spasm - I get them all the time, and tend to get them when I ride in the cold rain/large change in tempature (hot/cold). Everyone is different - so who knows...

Stretching helps alot. If you have one nearby, a trip to a hot tub will make you feel a ton better - I use one at the gym. Push-ups against the wall in a hot shower can also help loosen things up.