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Perhaps the prank was not well-received in that it caused Meredith to not only get angry, but also insult Amanda. Say some insulting things to her. Perhaps things which had been in the back of her mind, and when she was pushed to a point by Amanda, she just let all of those thoughts out.
And Amanda, thinking they were very good friends and not aware of any of these feelings that Meredith had about her, was very surprised. And very hurt.
You know, it's not hard to imagine....that someone you think of as a very good friend, and suddenly they say some mean things to you, out of nowhere. Because it would have been out of nowhere, as Amanda herself has said that she thought she and Meredith were good friends and there was no tension. And she has said herself that she did not know anything about what Meredith had told her British friends about her.
So perhaps Meredith said some mean things to Amanda in the moment, because she was so annoyed and so perturbed at Amanda.