Milwaukee Brewers minor league prospect Bubba Derby is a hero -- selflessly throwing his body over other people to shield them from bullets during the Vegas massacre ... and now he's telling his story.

The 23-year-old pitcher was attending the Route 91 Harvest Festival with family and friends when the shots rang out. Derby says he shielded two girls he met that night. His cousin threw his body over his girlfriend.

Fortunately, they all survived. The women have since found Derby on Facebook and thanked him for his heroic actions that night.

Derby says he's still haunted by the events -- and feels horrible for the people who didn't make it out.