Lily was feeling a bit groggy today, simply because all of her most difficult classes had been crammed into the early morning, and the night before had been a hard one. All of the girls in her dorm had caught on to some juicy gossip regarding the debacle in Diagon Alley, something about some hunky Gryffindor pulling a bunch of bodies from the rubble. Lily knew this was all false- James had told her everything about that day- but she didn't bother to burst her dormmates' bubbles. Besides, she doubted it would do much good. They'd turn on her instead.
So she had not slept well, but she was too determined to remain at the top of her class to let it affect her. So after her classes, the exhaustion began to set in. She sat on her own at the Gryffindor table, surrounded by people who spoke amongst themselves. Her fellow Gryffindors typically left her alone because A.) Because they knew her isolated tendencies much better than the other students in the other Houses and B.) They knew she knew where they slept, and that was enough to keep them from pestering her.
She finished eating and headed left, walking off to the restroom. She used Moaning Myrtle's restroom because Myrtle never whined at her. Once she had found out about Lily's father, she had mercilessly asked for information on him. In her second year, Lily managed the strength to inform Myrtle that Harry was long dead. It shut Myrtle up, and Myrtle actually developed a sort of kind nature for Lily, only ever asking about her studies.
And people rarely used the restroom, so she was confident she wouldn't be found there either.