Nevermind, we rescheduled.
Shit happens, and at most inopportune times. Reschedule and see how it works out.
Dude, at least she bothered to tell you. Yeah shit happens, you cannot predict things that can happen last minutes. I never cancelled last minute myself, but have had to do it couple times like an hour ahead or so, due to work issues etc. If she did that more than once, then there is a definite pattern or issue.
I think the consensus will be to let it go or give her one more shot. Things do come up on a daily basis for everyone...
I have a different reaction when this happens to me. I use the money to buy a nice bottle of scotch, have a glass or two that night, then make sure I keep some for the first of the month to savor while she's wondering how she's going to pay her rent.
It certainly isn't worth getting pissed over.