This could be due to a few reasons. One reason could be because of the increased blood flow to the working muscles during exercise, it diverts the flow of blood from the stomach which disrupts the digestion of food. Which leads on to eating before training. You should generally eat 2 hours before commencing training as this will allow your stomach to digest roughly half of the food eaten in the meal beforehand. You should consume water before training and not very much to none during as this will make you feel bloated and possibly vomit.