I haven’t had surgery yet, but if you are trying a lot of new foods or changing your diet your stomach might take a few days to adjust. I know for me, even when I would go on a diet and start eating less I would get heartburn easily. A dietician told me once that my body is used to producing acid for a larger amount of food and it needs time to adjust. If you aren’t used to the protein shakes (whey or soy) it may also cause a little tummy upset. Congratulations on getting started with the pre-op diet. When is your surgery?