Hiya
Usually for two reasons!
First is if you drink too much water in a too short time frame, so for example if you drink a litre in 20 minutes then it can cause it, therefore make sure you space out your water, little and often is best!
Second reason is that the packs are very nutrient rich and this can upset your tummy both initially and throught the diet, best advice is a diet sandwich of water, so have a big glass of water before a pack and then straight after to dilute the nutrients as much as possible, you might want to steer clear of the tetras if you have them as they are richer than the sachets.
As with all medical concerns if the problems persists then take a trip down to your GP and ask for his/her advice as they are the main source of medical advice, also make sure you let your CDC know.
Mike