Amplification of emotion, I'd say. Emotions tend to be stronger in dreams, as the portion of our brains that controls emotions is more active in dreams than when we are awake. In addition, the logic center is typically inactive, which is why we're not always lucid. Thus, any strong emotion in dreams can be much more powerful than in waking life.
Basically, you were frustrated and upset — more-so than you would have been in the same situation were it real — and blurted it out. Emotion-to-mouth, no logic center to get in the way to tell you not to say it.
That's my scientific two cents, anyway.

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