First thing you must know is that rejection is normal! It happens to everyone, if your single and meeting people at some point in time you will be rejected. It's ok that is the reason we approach and ask her out.
When you get rejected you can only act positive. Tell her it's no big deal and don't let your feelings get hurt. Why?
Because it's just one girl, there is another girl you can talk to right around the corner. Further you didn't know her. And lastly the reason you asked was to see if she wanted to get together, so if she doesn't she is just answering your question and being honest.
Let me share a story with you about my own rejection. It was roughly a year ago and just so you know I experience a healthy dose of rejection on a regular basis. But I also experience a lot of success too.
The way I see it is the more women you talk too no matter how great your game is you are going to have an increased dose of rejection. It is cause and effect, the more girls you talk too the more rejection you will experience.
There are some tools to help minimize this and when you understand how to talk with women you will experience much less rejection but the equation is still true! Back to my story I stopped by a friend's house, he was having a few people over to have some drinks.
I stopped in and had a drink and meet up with some friends because we were going out that night, and the host of the get together was staying there and entertaining the people that didn't go out.
Anyway I meet a girl and I talked with her for a bit. I was there maybe 15 minutes and talked with her for only a fraction of that time. As we were leaving I asked her if she wanted to get together sometime.
And she said "NO." I played it cool and said "that's cool" smiled at her and left with my friends. My friends were giving me a hard time but I told them at least I tried. And I would have regretted not asking her out.
About a week later I went over to my friend's house that hosted the get together and it turns out the girl that rejected me changed her mind and wanted him to give me her phone number. But I said she had her chance and she blew it, plus it's just one girl.
My point is just that this situation started out as a rejection from her and ended up as a rejection from me. The timing with dating is such a critical factor and sometimes girls are just weird.
It often doesn't make sense and you just have to roll with the punches and the best way to do that is to approach girls and talk to many different girls until you meet someone you really like.