A diplomatic mission. That's what she told her subjects she had spent all that time on. Aside from knowledge, what PB valued most in herself was her social standing, not so much as a royal of high standing, but as a lady and a scholar. Clearly one could see why she didn't want her subjects knowing of her "moment of weakness."
She didn't know how long she'd been in the library, and she didn't know how long it had been since she was dragged out by Finn and Marceline; one can never seem to keep track of time when they're not focused on the here and now. All she knew was that she was alone with her thoughts, and they were insufficient. Surely there was some redeeming detail, some justification for the bizzarness and morbidity she had witnessed?
It wasn't all bad; at least she'd fallen back into a schedule. She'd wake up in the morning, have a light brunch, and head straight to the library; there she'd multitask between further research and signing bills into laws; sure, it was a bit irresponsible, but what's the worst law the Candy Citizens could come up with? (She probably wouldn't have risked it if she'd given that question any real thought.)
What information she could find was useless; the first Prince and Princess weren't viewed very insightfully by history, rather like one views Achilles when reading the Odyssey or Merlin in the Legends of King Arthur; powerful and iconic, yet not the focus of the story. Genetics and mutations were a new science to the Candy People, a field founded by herself. She shuddered at the thought of comparing her heritage to the blasphemy against nature that was Lemongrab (which is not to say that other, more successful experiments were so vile. Old beliefs die hard, no matter how grounded in fact they are.) What remained of the Great Mushroom War were charred notes and hidden memories, locked away from her by madness and secrecy.
It was from this secrecy that she delved even further into madness. A strange paranoia, a hinting yet overwhelming mistrust surrounded her whenever she was around one of her friends that she knew to have even the slightest relation.
She had alienated Finn almost entirely for this reason, albeit subconsciously; Sometimes she would see Marceline, or at least something that looked like her; a fleeting shadow, a rustling tree.
She had cut herself off from her subjects as much as possible; where she once saw childlike innocence, she know saw animalistic ignorance; where she once watched with a motherly affection, she now watched in contempt and fear. No, not fear. Fear requires an immediate sense of threat. What she felt was a bizarre of her would kill to go back to the way things were, to unknow this forbidden knowledge; another part of her wished she knew more, understood more; and another part of her was incredibly anxious for her meeting later that day.