The flip side of travelers' diarrhea is constipation. While the old scourge of "tourista" gets most of the publicity, so to speak, constipation is almost as prevalent among travelers... and for some of the same reasons. No, it probably won't be the water, but it can be the jet lag and the diet that bind you up.
Being tired from not sleeping well on the plane, being dehydrated, the stress of travel... all of these may add to your problem. But wait, you say, I thought those caused diarrhea! Yep, they can cause both, depending on all the other circumstances, and depending on your body. What gives some people the runs, clogs up others.
It may be that you traded off your usual fiber-fortified morning cereal for a fiber-free continental breakfast... Too much refined bread and the cheeses that go with it maybe? My grandmother used to say, "Eat cheese, it's binding."
I used to love all those new breads and cheeses to sample for breakfast when we travel, but now I try to eat those "active enzyme yogurts" before we travel, and I look for yogurt and fiber rich foods at the breakfast buffet.
Our bodies are conditioned to the clock, and your body clock is all off if you've passed over several time zones....
When it comes, don't ignore the urge... you may be walking down the street in a foreign city and think, "Oh no! Not now!" Well, not going will just block things up more later, so do your best to find a toilet. Eventually your body will get on a schedule in your new time zone.
Remember to call it a toilet or WC. Calling it a restroom will only confuse people in many countries. And speaking of the facilities... Sometimes it can even be the strange bathroom facilities that cause travelers' constipation in some people.
Those facilities may not be as clean as you're used to... or you could encounter one of those "hole-in-the-ground toilets" instead of "the porcelain throne". Some people just can't go when the facilities are so different. I guess you'd better be looking for higher end (so to speak) hotels where things are more apt to be what you're used to.
When you go on vacation, you probably drink more alcohol than you do at home. Alcohol can dehydrate you, and that can make you more apt to suffer irregularity.
Drink plenty of water and fruit juices. Try to keep meals on a regular schedule, and try to eat plenty of raw fruits and leafy vegetables. Go for the prunes, granola, and yogurt if you get the chance at breakfast. You might want to try taking packets of a fiber supplement like Metamucil along.
And get plenty of exercise.... it'll help get things moving again.