It may be a moment of weakness while out having fun, she was going through a rocky phase in her relationship, she was caught off guard when you asked her for the phone number or has a difficult time saying "no" face to face, "in the light of day" she realized she really is not interested in getting to know you.....etc
The real question is: "Why should you give a second thought about someone who is clearly not interested in getting to know you?"
This type of rejection is a gift. It saved you time!
Be grateful she didn't go out on several dates with you leading you on and allowing to spend your hard earned cash just because she didn't have anything better to do.
Some men ask women for their phone numbers but never call them. This is especially true after drunken one-night stand. They ask her for her number so she doesn't regret it in the morning. Men asking for phone numbers and women giving out phone numbers to people they aren't interested in getting to know is usually a sympathetic gesture.