Am I overreacting?

I don't think you are overreacting at all. This sounds a lot like my former best friend. She was a user and negative. I also went on a trip with a friend once to Disney World, and she sulked and cried if we didn't make all her list of rides, although she goes every year. It was annoying. I was 25 and she was 24. Anyways, I would be mad too. My ex-best friend dropped out of me from a trip the day before, after paying airfare, tickets for the soap event, and she bailed because she has mental issues. I would honestly just get rid of her. You don't need toxic people in your life. You welcomed her as a guest in your home, treated her to lunch while you worked, and there was no reason she couldn't have sightseed on her own. She sounds spoilt too, IMO. I would just let her go. You don't need a user like that in your life. I've been there, and you deserve better, than to be treated like a doormat. Hope this works out for you!

"I sent a thank you email back for the flowers saying that I was really sorry that I ruined her trip, and she ignored the email. At first I guess I was kind of shell shocked about her behavior."

I don't know why you apologized for "ruining" her trip.

I wouldn't try to hash things out via email. Too much chance for misunderstandings. While in person would be best, over the phone is next best. Tone of voice can make a big difference in how a person takes a comment.

It does sound like you and she have gone in different directions in your attitudes and approaches to life. Before she started acting all pissy, were the two of you having fun? Was it like old times? When you were at your home/with the family (ie. your real life) vs sightseeing was she fun?