I read somewhere that adding a lot of fibre (i.e. taking metamucil) will help with jetlag and I've been doing it for the last year because I've noticed the same effect as the OP. It does help some. I start with a 6 capsules the night I arrive and reduce by 1 per night until I get to 3 and then call it quits. I take melatonin too.
I think it is a combination of all that was discussed above: 1) environment, 2) dehydration, 3) strange diet, 4) strange sleep patterns, 5) air pressure (I get really gassy on a flight.)
Sadly, I've noticed the same effect on North-South flights where the time zone doesn't change and lately, on long car rides, so I'll add another, 6) being folded up in a seat for too long stops things from progressing through the digestive tract normally.