Why is she ignoring me?

I have recently started chatting to a girl, and about a week and a half ago asked her if she wanted to go for a few drinks. She said she'd love to, but we've both been very busy so a couple of days ago we arranged to meet up today. I text her yesterday asking what time suited her and got no reply, then I text her again today asking if she was still ok to meet up, again there was no reply. I was starting to worry a little at this stage, so I sent her a facebook message just in case her phone was lost/broken. I asked her if everything was ok and also said (probably rather stupidly) that if she's changed her mind about meeting up just to tell me and I wouldn't get offended. I've had no reply to this either. At this stage I don't really care anymore whether she wants to see me or not, I just want to know why she's ignoring me all of a sudden. It's not as if I've said/done anything offensive to her, it's just come completely out of the blue. Should I just leave it, or wait a few days then ask what I've done wrong? I really want to know, but at the same time I don't want to be bombarding her with messages. I was even thinking of ringing her using a withheld number to see if her phone is working, then just hang up straight away. Has anyone else ever experienced anything like this?

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