There could be a perfectly reasonable explanation - she had a headache and was checking to see if you had tylenol? I would ask. She could just be a curious person, not looking to steal anything, just wondering how other people live. When I was a bored teenaged babysitter, I used to look around people's houses. I never had any ill intent, just curious about people.
Curiosity does not make it okay to invade people's personal spaces. That is NOT a reasonable explanation! When I read the OP, that is exactly what I read as making her feel violated - someone going through her personal spaces.
Not sure why people are jumping to stealing because OP didn't mention that at all.
If I am curious about someone I might look at their bookshelves, (or back in the day music or video collections on open shelving) but opening drawers etc. is NOT okay!