My boss told me to come in the next day when I wasn’t even on schedule.

I’ve been working for about a month in retail. I always take everyone’s shifts go early to work when the manager needs me to come in and I always agree. The thing is that she always asks me before if I can cover someone’s shift but this time she didn’t she just told me to come in to work the next day as she was leaving. What do I do am I a bad person for telling her I can’t go that day?