Oh sorry was on the bus home...I think that you kind of have some anger toward her right now so essentially forcing you to interact with her is a bad idea...I know I would have a hard time not yelling at her and you know she would see that as a threat again...

Your best bet would be to call and cancel..I really don't think it's a good idea to see her ever again...I would just call tonight and leave a message that you're feeling sick and won't be able to make it tomorrow...but yeah just call after hours and leave a message canceling...at that point they need to get in touch with you and you don't need to answer the phone...