Britney Spears -- Ex-Boyfriend Cheated with a Porn Star

Britney Spears didn't dump David Lucado for hooking up with any ol' regular girl next door -- TMZ has learned the other woman in question is a hardcore porn star.

Cali Lee is a 5 foot, 96 pound Latina who's been in the biz since 2009. She's famous for classics like:

-- "Itty Bitty Boobies"
-- "8th Street Latinas Vol. 20"
-- "Fresh Outta High School 14"

Cali is kind of a unicorn in the industry -- she's been in less than 20 scenes. Although, one of 'em was with legendary swordsman Ron Jeremy.

As for how David and Cali crossed paths -- we're told they met at a bar near Britney's house.

As we previously reported ... Britney kicked David to the curb after he was videotaped hooking up with Cali in August. Britney's dad bought the vid to keep it under wraps.