Last night Mariah Carey sang at the American Idol finale, and though she looked spectacular in an ornate white gown, everyone is buzzing about the fact that her performance was taped ... and that she may have been lip syncing!

Mariah took the stage for a seven-minute medley of some of her biggest hits, including "Vision of Love," "Make it Happen," "Hero" and her new tune "#Beautiful," and fans at the finale were quite disappointed that she didn't sing live. An X17 reporter at the show tells X17online exclusively:

    "Producers asked the audience to get excited for Mariah's performance, and then the screens dropped from the ceiling and they played a video of her singing - no one could believe it.  When the video was over, the crowd was supposed to cheer but instead everyone just sat there staring at each other.  And the judges' mood was beyond mild - they all seemed bored.  Mariah and Nicki [Minaj] didn't look at each other, and even Randy [Jackson] was unemotional about it being his last show."

And not only that, but Mariah acted like she was over it as soon as she walked in the building! Our source at the show added, "Hair and makeup primped her between every commercial break and not a strand of hair had even moved. She was texting the whole night and looked like she would rather be anywhere but there."

Despite fans tweeting that she wasn't actually singing, Mariah's rep told Entertainment Tonight, “No, she did not lip-sync! She sang 3x and even sang additional songs for fans in between [commercials.]”

Mariah just announced that she's embarking on a world tour, and based on how unenthused she was at the show last night, we're guessing it's only a matter of time before she announces that she's leaving. Randy's out, rumor has it Nicki is dunzo and even Fox exec Kevin Reilly couldn't confirm that anyone was coming back, as he told E! News earlier this week that "everything at this point is on the table, and we'll talk about that starting Friday."