Tiffany Struggles Through 'I Think We're Alone Now,' Adds 'F*** You' to Fans

80's teen star Tiffany is still on the road singing her hits ... but her latest show in Florida struck a sour note between the singer and her fans.

Tiffany was onstage Sunday night in Melbourne, FL with her band, belting out a few tunes including her hugely successful "I Think We're Alone Now." You can hear Tiffany struggle with a few notes, but fans help her out -- singing along word for word.

However, near the end of the song, Tiffany apparently hears or sees something she doesn't like in the crowd ... telling them, "F*** You!!!"

A rep for the singer tells us Tiffany had lost her voice and got frustrated with her performance. It's clear in the video, she was struggling to hit several notes during the gig, so it's possible some concertgoers were heckling ... but Tiffany just wasn't having it.