She may be a person who is uncomfortable saying more than "I don't feel too hot."
She can feel sick and cancel and isn't required to prove it or need emergency medical care. She may have health issues that aren't reflected in her voice. She may have hoped to have been feeling better and not have to cancel, but it turned out not to work out that way.

It would be really helpful and very kind of you, if you could accept that she felt poorly enough to cancel a session. It's a good sign that she takes care of herself, because if she doesn't take care of herself, then she can't be of help to others.

Your session is important and her feeling sick and cancelling doesn't change that. And I'm sure she'll understand when you tell her how her cancelling, especially at the last minute, can make you feel upset and can affect your ability to trust.

I am really sorry that your appointment didn't get to happen.
When will you and she meet again?