We usually miss routine things (swimming lessons etc) if they clash with a birthday party, even if the kid isn't a close friend. I know every one is different, but I would be inwardly peeved if someone passed up an invite to my child's party to attend something they do every week.
I try to get my kids to attend every party they get invited to just because I know how anxious I get on my kids behalf when sending out invitations and worrying that no one will come.
So I can see why she'd be a bit shirty about that but not the playdate thing. They're no big deal so they just have to be fit in amongst everything else you have on.