I see two issues:
1. The horse wasn't safe for you. Based on what you said about how things were going, if I had been your instructor I would have taken you off as well simply for your safety. In fact, I've done this before as an instructor, and as a rider it happened to me when I was younger. Most instructors will try not to overmount their riders, but sometimes it happens since horses are most definitely not robots.
2. Your lesson was cut short. This part is not right, unless she was actively teaching you something while you were watching someone else ride the horse. In this case, I don't see a huge problem with it, as long as it was understood when you began taking lessons at this barn that some of the learning would be out of the saddle. However, if they promised you x amount of time in the saddle per lesson, then you might say something to her.
Don't feel like a bad rider or be embarrassed. It's really not as uncommon as you think. Be grateful that your instructor decided your safety was more important than anything else, chalk it up to a learning experience, and keep riding. If you keep working hard, eventually you'll be able to ride that horse again with no problems.