I think that every part of her is in fact invisible. And anything inside of her is completely surrounded by her invisible body, so it becomes invisible too in a way.
I assume it works in the same way that anything in a chameleon is obviously hidden and doesn't interfere with it's camouflage, but instead of being camouflaged she's invisible. of course, that would mean that anyone looking inside of her mouth from the front would see her food whenever she opened it, which is slightly funny to think about.
That would also explain why the sweat is visible, it's made up of things that aren't her body. Water, salt, etc. so her poop and pee are also visible, probably.