My female pekin duck is getting really mean. she bit me and left marks. what is the matter with her?

I got this duck from someone who had hand raised her, and I try to treat her more special than the other birds. She goes and attacks my mom, and when I try to walk, she steps on my feet to stop me, then starts trying to bite/peck my foot, often succeeding in untying my shoes. My mom thinks she is PMSing, but I don't think it would last that long if she were. She also doesn't like water. At first, I thought it was just a sense of vertigo from not being able to feel the ground, but then I got her a shallow kiddie pool, and she still won't stay in it after I walk away. Yumyum (my other duck) doesn't act like this. He was the first duck I got, and he is really mellow, so I took his behavior to be normal. Are female ducks just this mean naturally, because she didn't act like this when we got her. She had never seen another duck before we got her, so she might have just realized that she is one. That doesn't explain her new behavior, though. Can you?