Law enforcement sources tell TMZ ... Denise was found unresponsive Sunday on the floor of a friend's home in San Diego. First responders arrived and attempted to resuscitate her, but she eventually was pronounced dead.

We're told drug paraphernalia was found at the scene. The cause of death is still pending, and San Diego PD is currently investigating.

Fans of the 2011 reality series remember Denise was part of one of the three couples on the show that followed the lives of extreme sports athletes and their partners. Denise was dating professional skateboarder Pierre-Luc Gagnon at the time. They had a son together, but had since broken up.

Susie Stenberg, Denise's friend and former "The X-Life" castmate, tells us Denise, unfortunately, was homeless and living in her car when she died ... adding Denise reached out to her just the night before to tell her she loved her.

Susie says Denise was "the most loving and loyal person."

Denise was 44.