Overtime

Hi all I work in a busy supermarket I'm part-time and work 12 hours on a weekend rotter. I've just finished a 6 day overtime, 8hr shifts each day without a days rest. I'm a little annoyed, as they called me to ask me to come in on my week off. I'm busy spending time with my family so of course I said I can't this week. I just wondered if I was in my right and will it look bad on me? As I'm only a temp but I didn't sign up for week days. Plus I've already done more overtime than the other temps that I started out with.