Can you help me come up with excuse for legitimate situation. Please help/advise.?

So my boss asked me to work on Thursday with a note but I didn't reply because here's the honest truth: My boss would work the same hours that day and I've been feeling depressed and I don't want her to see me being depressed and I have a psychiatry appointment the day before to get meds but I don't know I will react at work if I've never taken them before so I need to tell her something other then I have a shrink appointment so that she still gives me more hours later.