I whitened my teeth with Crest White Strips Professional Effects and after just a few days I stopped they started developing a yellow color and are now yellow again. Why does this happen so fast? Is it my own health problem? Or crest white strips problem? I don't drink much tea or coffee though I have consumed some these few days but it shouldnt have such a large effect. I brush them day and night with a whitening toothpaste but they just get yellower and yellower each day.