Agree with CrazyEight.

I had a 3 year old girl before that would be totally fine all morning, then as soon as she knew it was nap time she would start screaming and crying for her mom every single day. The only thing that worked was to acknowledge her feelings (you will see your mom soon!) and then be firm with her that she needed to stop crying and go to sleep or that she needed to go play now. She didn't end up being at my daycare that long, but by the time she left it was much better.