iPhone 3gS saying "Charging is not supported with this accessory"

I just got a brand new iPhone 3gS two days ago. It synced fine but now everytime I plug it in I get the error message "charging is not supported with this accessory". I'm plugging it into the Apple charger for the iPhone. When I plug it into my computer and the wall charger, both using the Apple usb cable, I get the error message. When I plugged it in to charge last night it was working, but when I woke up my phone was not fully charged and the error message was on the screen. Please Help!