killerman,
Calling her out on it never really does anything good for you, so I certainly wouldn't do that. It seems you're aware of that, though!
You can probably make a decision here. If she seemed like a girl who was genuinely interested in seeing you, then I would give her the benefit of the doubt, but I would probably wait a week or so to set up the next date. Don't make it seem like your time isn't valuable by attempting to immediately set up the next date as if you have plenty of time for her. (EDIT: Text you might send here would look like this: "No problem! Stuff happens. We'll take a rain check on it

")
However, if you felt like this girl was very "on-the-fence" about you and that her excuse is likely not legitimate, then respond with something rather nonchalant that has an ambiguous tone (i.e. "it's cool, no worries") and stop contacting her. Sometimes this can be enough to make her wonder if she should have taken that opportunity with you (without sounding whiny or upset), and you might hear back from her within the next few weeks. If she suddenly contacts you out of the blue and seems interested in meeting up, then arrange another date with her quickly. It's
very likely she won't miss this one!
- Franco