Ahhhhhhhhhhhhhhh Dont U Do That, Smile It'll B Ok Cause He Was A Douche Anyways: MILEY CYRUS CRYING IN VEGAS DURING 1ST POST-LIAM PERFORMANCE!

Miley Cyrus broke down crying in Vegas yesterday ... while she performed a song rumored to be about her break up with Liam Hemsworth.

Miley twerked it up on stage at the iHeartRadio Music Festival -- but the tears started flowing hard once she began singing "Wrecking Ball."

As we previously reported -- the couple has called it quits ... and Liam has gotten tight with telenovela actress Eiza Gonzales -- who BTW is crazy hot.

Despite the cry-fest ... Miley cleaned up for her second performance of the night.

And wore this.

I'm sry but this outfit is so f-ing ignorantly ugly, looks like its outta season by "Never". Plz Miley ur a freaking pretty little "Rock star", but ur f-ing it up by wearing ugly azz clothes like this! 

P.S. THROW THAT POUR OUTFIT AWAY ASAP, U MITE AS WELL CAME OUT NAKED. AT LEAST IT WOULD HAVE LOOKED BETTER THEN THIS PIECE OF CRAP!