Why is a tire pressure light on in my Scion tC?

I just filled up my tires the other day to what they are supposed to be (32 front, 29 back). I drove home and the car had been fine the whole time without the light on. My parents just took a trip to Florida and when they came home they said the light was on, so I figured my pressure was up too high. I let the tires cool for a few hours and I just now checked them and they are all right where they need to be. What could be wrong? Should I take it in to a repair shop?