It could be dehydration - my brother used to get really ill with it whenever we went somewhere warm, he once announced his arrival at a hotel by throwing up over the receptionist lol. We used to take salt tablets, they make you drink more and they really do help.
Just follow the basic precautions - eating food that's been cooked in local water should be ok, as most of the bacteria will be destroyed by boiling. Improper refrigeration is a much bigger problem, so just make sure that what you eat has been stored properly (as far as you can, obviously). If you're eating out, go for hot rather than cold food. Avoid things like fruit and salad, as these may have been washed in local water (fruit you peel is fine) and don't have ice in your drinks. Brush your teeth with bottled water, and when you buy bottled water make sure that it's 'sealed' - some places just refill the bottles with tap water. Also, if you're swimming a lot, make sure the pool is clean - a lot of people get dodgy tums from dirty pools rather than dodgy food.
Make sure that you keep hydrated (the salt tablets will help with this) and make sure that you're getting enough salt and sugar, as these will help you rebalance your electrolytes, which get screwed up when you've got vomiting and diahorrea. Take Imodium (or Kaolin and Morphine - similar stuff but cheaper, but it tastes FOUL) and try to eat vegetables and wholemeal bread rather than fatty stuff, as this can irritate your stomach.
I doubt you've got IBS, it's probably just a combination of factors - my brother went out to Sri Lanka and most of the people he went with got ill, some just got worse than others.
Hope you feel better soon!