That seems like a totally reasonable and probable reason to me! I'll buy it.
And to the OP, I thought she "abandoned" the nest. A little time off once or twice a day is perfectly normal and necessary! I know, your first broody doing that is cause for panic, luckily she knows more than you
did. She taught you something!
I my years of experience with sitting hens, any brood hen of mine who comes off the nest everyday to either eat, drink, evacuate, bathe or all four, has a pending date with a pressure cooker. While I believe that a hen knows best how to hatch eggs via body heat alone, it is STANDARD OPPERATING PROCEADURE for a brood hen to sit tight for at least 48 hours at a stretch and more typically for 72 up to 98 hours without leaving her pending baby chicks unattended. The simple fact is that a brood hen who spends too much time off the nest is not a serious setting hen and she will almost surely let both you and her clutch of eggs down at the most inconvenient moment.
As ambient temperatures increase, hens do tend to stay off the nest for longer periods. Don't ask me how I know this or how long to expect a hen to perambulate about, I can't read a chickens' mind, I can only observe their actions and compare the chickens' day to day actions to current conditions.
Don't worry about whether your setting hens bottoms are getting sore, they are good at doing what they do as long as we humans provide the resources a hen needs to do her job. These resources include a safe, dry, and private maternity coop, a sheltered and dark nest that apes a natural chicken nest, plus food and water. If you are unsure if your brood hen is eating or drinking, your nose will tell you.
As justplainbatty mentions, when a brood hen evacuates her bowls she will expel a chicken poop of gigantic proportions. Don't let your children see this setting hen poop because your kids will think that you bought them a pony.
You also will not need to search for this setting hen poop, just let your nose inform you that your hen has come off the nest, the smell will tell you that she did and if you're like me, this smell will stay with you for life.