It's better if she's honest than getting whatever she's got.  I had one gal cancel on me last week because she got pink-eye.  I was glad she told me because I was going to visit relatives with kids and really would have felt bad if I'd contracted it and given it to them.
Today, I wanted to see a new gal but she said she was a bit under the weather, so we're getting together tomorrow.
Now, are there times when a gal dumps you because, say, she got offered a multi-hour session but didn't want to say that?  Sure.  So give her the benefit of the doubt and call someone else.  Or go to an agency.  They're great for last-minute action.