Elsa's able to convince herself everything is okay. Anna seems okay... Surely she didn't expect Elsa to react any different?
Except... it becomes obvious that she did. They get home and Anna quietens. Begs off dinner and goes to bed early for a class she doesn't have. And Elsa lets her be because this is rejection. And it's hard and it's painful.
It's not until she walks past Anna's bedroom, hours later, that she realises just how painful.
That she realises just how hurt Anna is.
And she really shouldn't be surprised.