Why does my manager schedule me so much?

So, I work in fast food. I recently had to change my availability for January because I will be taking more classes then. My manager asked me about this, and I told her that in January, I wouldn't be able to work until 2:00 p.m. on Tuesday and Thursday. Then she said, "January? You put it for this month. You also didn't put a note on there either." Which I absolutely DID! I also made sure that I put the specific date. But I let it go, and just corrected her. Now, I had already been scheduled for next week, where I was only working four hours on Tuesday, and she had given me Thursday off. However, after she found out that I WOULD be able to work, she scheduled me for 30 hours next week. I am EXHAUSTED. I was really looking forward to next week because this week, I've worked every single day of my availability, and because we have been shorthanded (missing at least two people) I have done the work of what is supposed to be done of at least three people. (and I have to be at work in three hours.) I am so stressed I can't sleep. I am also a full time student, so not only am I working every day of my availability, I have school or church on the days I am not working. Am I just being lazy? Or do I have a reason to complain? Why does she schedule me so much?