What does it mean if a girl cancels our date because of her 'period'?

Last week, I (F17) asked out someone (F16) on a date to watch the new Superman movie, and she agreed. Mind you, this was on Saturday, and we planned it to be on Thursday. We were texted back and forth almost constantly about times, seating, and whatever, and we've both really looked forward to it since the decision. I have also gained a genuine interest in this girl, and we've learned quite a bit about eachother just through little "good morning" texts. However, this morning I asked her how she was, and she didn't respond for another hour (which is very unusual, as it usually only takes her about five minutes to even read the message). And she did not respond with her cute little "I'm gooooodddd", and instead completely changed the subject by CANCELLING THE DATE, using the excuse that she just got her period. I told her that it's completely fine, and that I'm on mine as well, but she continued and said that she didn't want to ruin the mood. We both decided to postpone it, despite how disappointed I was, considering how excited she seemed, and how excited I was. What I would absolutely love to know is why was she not willing to go through with the date anyway, like I was? And perhaps was she just lying and just wasn't interested in the date?