Staining during whitening treatment?

I followed a whitening treatment with opalescence 16% for 12 days. During this period I stopped drinking coffee, wine, cola and tea. on the other hand, I drank hot chocolate a couple of times, beer and ate chocolate. Is it possible that this may have affected my final results? I still have yellow spots on my teeth and I don't smoke. I used 4 seringes. Is the white diet really a must during a 2 weeks whitening treatment? Thank you.