Sounds like a lack of communication on both parties behalf. Ask to have a meeting and discuss your current schedule, and what you need it to be. Have a written page of concerns so you don't get sidetracked and forget anything.

What your boss does isn't your concern really. If she goofs off on FB, that shouldn't affect whether you are doing your job. You missed a meeting with her, and now she missed a meeting with you. You agreed to extra hours, but now it's too many hours. Maybe she thinks she's been doing you a favor giving you extra hours, and it seems out of the blue that you don't want them now.

It's never one-sided. Get a sit-down meeting to discuss #1 your schedule, and #2 your concerns on being written up, and how you can avoid any future issues. It's your job, you are supposed to work WITH the people, including bosses.. not work against them and opposed to them.

Yes, she could refuse to work with you, but it doesn't mean YOU shouldn't make the effort. You can't control her, you can only control YOU.