Female Cockatoo scared of males

Hello,
   
I have owned my sulphur crested cockatoo for just over a year now (i have had feathers DNA tested and she is a female) All my family have a very good bond with her, mine being the closest. She was always very affectionate towards my father until about a month ago. When he approaches her or even walks past she gets very frightened - her crest goes up, heart beats fast and she tries to fly/run away. there are no signs of aggression and she seems to be doing this to all males now... Please help! she used to be such a happy bird!