I just went through the same problem. My tire pressure sensor light would come on almost every 6 to 7 days. I would check the tire pressure and find that all of the tires were very close to the correction p.s.i., but the driver's side rear was down to 25 pounds from the last time I filled it to 33 p.s.i.

I took it to a tire dealer and they found a small nail in center of the tread. It's been a couple of weeks now and the tire seems to be holding its pressure. I'm betting you have a small nail or something in your tire or you have a leaking valve stem. Definitely get your car to a tire shop and get it looked at.