Ok let me see if I can explain. I work pretty much all the time. Even now, I am working while posting. So, if I make a date with someone, I have to move a lot of things around to make time for the date. So, lets say I am in the OP's "shoes". Ive made a date with someone, rescheduled everything, made reservations, etc. Then, just a few days before the date, she calls and says she has met someone and cancels the date.
Now, the term "I have met someone" implies a recent event. Did she meet someone and is suddenly engaged? (If it does, it means she really is a flake). Why would meeting someone automatically mean she must cancel our date? Cancelling, when you don't know the trouble the person has gone through to arrange the date really is rude. It is insulting when someone automatically assumes that the person she just met is somehow better than you, without having gotten to know you.
Ive gone through something similar to what happened to the OP. Only, I had already met the woman, just not on a date. We had met at church, became, well, I wouldn't call it friends. Acquaintances. I asked her out and four hours before the date she calls and tells me that she had met "the perfect guy" the night before at a friends party. I'm insulted, but whatever. Anyway, two years later she comes back to church...with a toddler in tow. Seems her "perfect guy" ran off 3 months after the baby was born.