Soo mine's doing the same thing, you're not alone Doug, lol! Did some spot research on google and like delphinus said, nems can be either male or female, releasing either eggs or sperm into the water to propagate, much like corals do in the wild (and rarely in our tanks).
I think mine's doing it because he, or I guess I should say she thinks she's dying =/ "She" got food poisoning a while back and has been bleaching since and unable to retract "her" stomach.