I just started a new job and i already have an issue.?

I just got hired by a grocery store in the produce department. I start tomorrow after school, working 5 hours. I do the same on wednesday and then again on saturday I work 7 hours. My problem is that during the interview, I only asked for 10-12 hours a week or 2 shifts. They've already thrown that out the window. I don't wanna ruin the job when i haven't started yet but I need to ask for less hours if i'm overtaken over the next two days. I need this job though to get university money and it has taken me months to find one. now I dunno if i should ask or not. God, life is so stressful. Plus i gotta study for my calculus test tomorrow...yay. Help me!