Yup, her behaviour seems downright weird to me.
The only thing is, did you ask her in front of anyone? i.e. was there anyone else in earshot? Maybe she liked the idea but was just so embarrassed about other people knowing (which isn't great either, to be fair) that she turned you down, and now can't face you because of being doubly embarrassed.
Either that or it was, indeed, for a bet.
Either that, or it was someone else's phone number that she gave you (in a "my best friend fancies yewww" way), thinking it would be obvious whose it was, and it's all been a horrible comedic misunderstanding.
Whichever of those, or whatever it turns out to be, it is indeed incredibly "playground". How old is she, if I may ask?
And yeah, from now on best to just ignore her, unless she sends you a message apologising and giving a credible explanation. You don't want such people in your life if you can help it.