Law enforcement sources tell us ... Caserta was attending a class at a school in Sunrise, FL when she suddenly fell forward at the desk where she was sitting. Her alarmed classmates immediately called 911.

We're told Cristy went into cardiac arrest as emergency responders arrived and administered CPR. She was transported to nearby Westside Regional Medical Center where they continued efforts to save her, but she was pronounced dead.

Cops say they do not suspect foul play and no drugs were found on Caserta -- it appears she died as a result of the sudden seizure. An autopsy will be performed Friday to determine the official cause of death.

Cristy appeared on Season 15 of "The Bachelor" ... the one fans know as Brad Womack's second shot at finding love. She was sent home the first week.