Oh Bluebell, I do feel for you honey, I really do. I could have written your post a couple of weeks ago.
YANBU i also had a mum do exactly the same to me: What are you doing at the weekend? I had plans sat and sun afternoon and said what I was doing thinking,that if her kid did want to do something then it was an option she could meet up with us. (Her DD ia always inviting herself round despite never playing with my kid at school or in the break and having made various nasty comments in the past).
She then says What about evening and then asks could her DD sleep over?! This was with 2 days notice despite knowing my DH works ridiculous amount all week and I am 31 weeks pregnant...
I said I'd let her know the next day after speaking to DH - my biggest problem being my house is a shit tip at the moment and i hate sleepovers and i am knackered - the next day I said yes okay (knowing I'd have to pull an allnighter to clean up)and she said very offhand oh i'm going to take her with me.
I think there are always people who will not think and there are others incl on MN who, because they can keep their houses clean, have no problem with spontaneity and cannot understand others who have mess pile up (put away your judgey pants people - it's not always as cut and dried as you think!)
I have also had the worry of has a message been received...have been chicken shit enough to get DH to ring and text that i was ill and collect DD from school. On another occasion I kept my child off to avoid a playdate (yes I know...but these are 2 incidents in 4 years, normally i bust a gut to make the playdate happen!)
Sadly some people don't take no for an answer in a bart simpson stylee and others think nothing of imposing whenever it suits them. Plus there is the added guilt of playdates being a healthy and sociable part of childhood - were it not for that I would avoid them entirely particularly when not reciprocated or when a mum takes the piss with duration/pick up times etc
Good luck tomorrow bluebell! xxx