Why does my iPhone say my charger is not supported with this accessory?

If tried canned air changing my charger check for iOS updates but know change I'm starting to think my my charger is not compatible with the iPhone 3GS

This is a common problem with iPhone chargers. Apple makes them so that they break easily so that you have to buy more. The only solution is to buy a new charger. You can probably find one cheap on amazon. On the new iPhone 5 charger they made it even worse, they break almost once a month. Don't get another iPhone, they're over rated.