When Ben was in a fight with Vilgax, in a prototype genetic modifer ( so vilgax could build an Omnitrix ) ben's dna got scrambled and he became a girl. He unlocks Genbig ( who is a blue female version of way big ), Boogeygirl ( who is bogeyman when he is ben ) and several others. He then becomes Ben again ( through a cure made by Grandpa Max) but he can become Gen again at any moment. Vilgax did not get the omnitrix.