Something similar happened to me. When I came out to my mom, she seemed to think for some reason that I had convinced myself that I was gay.
She thought that I had read about homosexuality on the intarweb and thought to myself, "Hey, that sounds cool! I think I'll try that!

"
I had to explain to her that it doesn't happen that way, and while some people are confused, I was not. I made sure she understood that I wouldn't have said anything to her unless I was completely sure.
After this, she played the old "but you've never been with a girl" card. This kinda pissed me off, and after gaping at her for about a minute and a half, I said "Mom, tell me. Did you have to have a penis inside you before you knew you liked it?"
Silence.
Anyway, that was over a year ago now, and she's since become very accepting. Of course, whether or not your parents will come to terms with it (and how long it will take for them to do so) depends largely on their upbringing and their past experiences with gay people, etc.
You may just have to give her time.