It could be the sign of a dying alternator. Get a voltmeter and use it to test the battery. With the car off, you should get around 12V. If its much lower, the battery might be dying. With the car running, you should get around 14V. If its lower, but the battery tests normal, then the alternator might be on its way out. In either case, you might want to take it to Autozone or somewhere where they'll do a more comprehensive test on your electrical system.
