Dev -- My Sex Tape was from 'A Long Time Ago'

Dev -- the singer behind the smash hit "In the Dark" -- is desperately trying to block her sex tape from going public ... claiming the mystery seller is just "someone trying to embarrass me."

We broke the story ... the footage is being peddled to various media outlets ... showing Dev with an unidentified male.

Now, the 23-year-old singer tell us ... “My boyfriend at the time and I were young and having fun, it was a long time ago."

She adds, "I am now a mom and engaged and have more important things to think about then someone trying to embarrass me.”