she prob needs time to digest it. some reassurance that you arent in love with her may help...silly as that sounds to you, she might be thinkin you are attracted to her. good luck.

Give her a little time, and then if she's still hesitant around you, just talk to her about it. Maybe she's doing that classic homophobic thing, you know... 'She's only telling me because she wants to be with me'. Come right and tell her that's not what you want. Just because you're gay doesn't mean you want to sleep with every woman you see, just like her being straight doesn't make her want to jump in bed with every guy she wants to see.
If she's still uncomfortable around you after that, try to explain to her that you're still the same person you've always been. You just don't like boys the way she likes them, and that difference is just an intrinsic part of who you are. If she wants to continue a friendship with you, she has to accept that part of you and understand that you're just trying to be happy the only way you know how.