Don't worry, though -- Google says it's likely just a temporary problem brought about by something as ordinary as charging or passing by a metal beam. You can easily recalibrate your phone by holding it in your hand and drawing the figure 8 in the air. The animation below should give you a pretty good idea how to do that. You might look funny doing it in public for a bit, but if you can endure people looking at you for a minute or two, Google says it will fix things right up.