If you were going to be home over the weekend anyway, then your mom would certainly be ok asking you to take care of the dog.

I do think she should have asked ahead of time, though, and not just assumed. And once she learned you already had plans, she should have made other arrangements.

OTOH, you are an adult, and living in your parents home (I assume) rent-free. Technically, your parents could have expected you to make it on your own dime seven years ago. So since they are still providing for you to some degree, I do think you owe them something, too. Maybe not this particular weekend, but in general.