Whitening strips generally contain some kind of caustic bleaching agent. If you have other chemicals from your toothpaste in your mouth and on your teeth, there may be some kind of adverse reaction between the toothpaste and the bleaching agent.
According to the (Australian) Herald-Sun website, the Australian Competition and Consumer Commision warns against using Teeth Whiteners. Because of the chemicals contained withing the whitener all sorts of unpleasant effects can occur.
