could be your stomach reacting to the strngth of the vits and minerals, try having a glass of water before and after your shake. if it continues more than a couple days then speak to your cdc about what to do about it.
oh as another thought any chance you might be lactose intolerante? could try the lactose free packs and see if that improves it
happens for a few weeks to me too. it's the concentration on vits and mins. general advice is to halve your packs. i just put up with it. the more water i drink in the early days the more it makes it worse. used to happen loads at the gym after drinking a couple of ltr of water.