hi Jen
it's very very common for these kind of unscheduled naps to throw your day off- if I am ever travelling near nap time I have to try everything to make sure dd doesn't fall asleep or we get exactly the same. for example, now she is older she is on one nap a day- last week we were travelling near nap time and she fell asleep in the car for 3 minutes- and completely refused any kind of nap the entire rest of the day. Grrr. In her mind she's had her nap, it's done, and nothing is going to get her back to sleep when we get home. Sounds like your dd is just the same.
I guess the advice would be either- try everything in your power to make sure she doesn't fall asleep, or, if she does, then you'll have to kind of go with the flow for the rest of the day- keep her up for a while before trying for a proper nap, but not as long as you would if she'd had a decent long nap, and watch her cues to see when she's getting tired. It's really hard, I know.
hope this helps a bit xx