What did you ask her?
Also yeah a girl you're chasing saying "You're a nice guy" is a bad thing unless the girl herself is into "nice guys" which is unfortunately very unlikely
What you want is to be nice to her friends/girls she knows...overly nice if anything but a dickhead to her. I personally hate doing this as I am a nice guy, I enjoy reactions from people that I'm nice too and enjoy cooking/doing shit like that that makes people cheer up. However I learnt a long time ago that.
Nice guy that cooks/provides for a girl without asking for sex = The girl WILL 99.9% TAKE THE FUCKING PISS. (Obviously circumstances completely change when the girl acknowledges that she is your girlfriend. Cooking for the girlfriend/doing nice shit has always ended for me with sex that involves awesome lingerie and shit. I surprised an ex once with home made lasagna + extras, she disappeared off home and came back wearing only a massive coat and the best lingerie underneath <keep that in mind for your next girlfriend...aka you don't need to game them.)
Dickhead to her/nice to everyone else = She gets confused and wonders why your a dickhead to just her. blahdeblah, she thinks about you and attempts to get into your good books anyway she can aka sex. After sex you treat her like a princess.