while she was on a date with another guy.
.....So you 'assumed' she was on a date with a guy? How do you know it wasn't her brother or cousin or maybe just a friend?
She probably thought it would be rude to converse with you whilst she was with her male friend/brother/cousin or perhaps she was a little shy to approach you for a conversation. She obviously was interested in you and didn't want to experience the "Ships in the night" thing, so she slipped you her number.....Call her and find out what the story is then go from there. 