Sometimes you gotta poop in the McDonald’s bathroom despite everything you know about life.

Having bowel movements as infrequently as a few times a week is still considered healthy. Perhaps not as healthy as daily, but that isn't the point of this discussion. Doing the speed limit in most areas of North America means this trip could be done in about 14 hours, plus fueling time (I've done a similar trip in 10). If she sped just 10% over the speed limit, which is generally considered little enough to not attract the attention of the police in my area, she could have done it in less than 13. Not only is it conceivable she could have done this without having to poop, if she managed her fluid intake, she quite possibly could have done this without having to pee.

None of this has any bearing on whether she did poop or pee, but she was an astronaut, and astronauts are known for their ability to plan ahead. And I suspect any denial issues she had (what else is going to make you think continuing a relationship with a kidnapping is going to have a happy ending?) didn't revolve around her waste elimination processes. So she could very well have worn undergarments to deal with those problems as a precautionary measure and just not used them.

The diapers might have been for the target of her little roadtrip rather than herself, because the other things she packed sounded like a shitty kidnapping plan.