Pop icon Lisa Lisa is opening up about her secret cancer battle in her early career -- revealing she kept it under wraps for years out of fear it would cost her everything.

The legend dropped by "TMZ Live" Monday to chat about her new Lifetime biopic, "Can You Feel the Beat: The Lisa Lisa Story" -- telling us she was already concerned over industry folks asking questions about her swelling and fatigue as she battled breast cancer at just 21 years old.

Lisa reminds us it was the '80s -- a totally different time for medical tech, even public perception -- so she feared speaking up would leave her with nothing. In fact, she kept it so under wraps ... even her mom didn’t find out until 16 years later.

As we know, Lisa’s health never slowed her down -- she went on to inspire a whole generation of Latina artists with iconic hits like "Can You Feel The Beat."

Check out the clip ... LL also dives into the filming process for the biopic, where she's played by actress Jearnest Corchado, calling it therapeutic -- even if it led to 3 major breakdowns along the way!