Ladies... do you think I have a right to be upset/disappointed.

About 2 months ago, I told my mum we have plans this weekend and could she please take George overnight. She said yes etc etc. This morning she calls me and tells me she cant take the night off work tomorrow so she cant look after him.
I can accept that she cant do it, but bloody hell, couldnt she have worked that out earlier than the day before?
I was so looking forward to a day/night off, and now I am so very disappointed. When she called I didnt say anything, but now I wish I had because I am annoyed.