Constipation is usually due to not enough fiber in the diet and dehydration. Flying, especially long haul, can cause people to change their normal routine. They eat differently and don't drink enough water. Also, physical movement actually helps stuff travel through the intestines.

Next time make an effort to eat enough fiber rich foods, stay away from alcohol during flights, make sure and get up and move your body around, and increase your water intake. Chances are this will help tremendously in the way you feel.