Ugh. I offer my sympathies. She seems like kind of a cow, in general.
But I am not sure I would say anything at this point. She doesn't really have an obligation to tell you anything, and as long as she is sticking to her end of the bargain (in regards to paying fees, giving proper notice, etc), you don't have any issues other than her questionable character!!
If she does ask about that 5th day, I would tell her "Sorry, I have that space filled". Too bad that she was here first; the agreement was for 4 days a week, not 5. Be prepared to fill that spot if she does take off, because I bet she will....