It's possible she was interested at first and has now lost interest.
Attraction has an expiration date.. you probably aren't still as attracted to your high school crushes now, are you?
When a girl has a boyfriend, her window of attraction is much shorter- you have a limited amount of time to seduce her, or she will begin to doubt seeing you.
The last guy to have sex with a girl has the most power over her. I have lost girls to their other guys by not moving fast enough- the other guy got there first and she lost interest in me. You have to move faster- try to set up the date for within a day or two of meeting the girl.
Never close the door on anything- she may still want you, but you have to push through her extra high barriers to entry
You should ideally have multiple women you are pursuing at once- this makes you less needy, and makes you less invested in this one girl.
May I ask if you paid for the date? I am a believer in not paying for the first date, and only paying for dates later in the relationship after she has done some favors for me
I'd say just keep pursuing other girls-I think the window of opportunity has probably passed with this one.
It's a learning experience- when I was first starting out, things like this happened to me all the time, and now I know how to handle them better.