Why do I feel the need to lie about my PND?

[BluePop]

I have just been diagnosed with severe PND, I had to see an emergency MH team, who decided I wasn't in any immediate danger but am seeing a CPN next week.
My GP has also put me on Citarolpram (sp?) and made me an appointment for a week's time.
My problem is this- I am on ML at the mo (I'm a teacher) but in my spare time I make and sell jewellery, and sell scarves, make up etc at craft and gift fairs.
Next weekend I have booked for a MASSIVE craft fair near where I live, and I have been getting stock together for months.
But I just know it's a very tiring thing to do (it's a three day thing) and I just can't face it right now. I'm definitely not going.
The organiser has been on at me for the balance of my stall fee (it's £150 and I owe her £100).
I need to phone her today to cancel, but can't bring myself to tell her the truth. It's a big deal and if I just don't turn up, I will lose the chance to go there again (there are 3 a year, and I stand to take hundreds of pounds, if not more).
But just saying I'm seriously ill sounds like a cop out - if I had pneumonia or a broken leg I would feel fine with it, but I had bronchitis last week and was genuinely disappointed when it got better because it took away my excuse not to go.
But why do I feel I have to make one? Is it reasonable to say "I'm not very well" and leave it at that, hoping she doesn't think I'm making excuses?

[tatyr]

if you don't have anyone who could go and sell your stock on your behalf, i would be inclined to say "I'm unwell and have just been started on medication, i need a while for it to work. if questioned I would say i had PND, (which i do!) i doubt you'll encounter anything but sympathy.