I agree with Kevinshorses, it wouldn't hurt to ask. Also, you say they have a good reputation, it isn't so much the thought of them "ruining" her that I meant, it's more like it only takes 1 bad experience to cause an issue like this, especially with sensitive types and it isn't necessarily the fault of the trainer either. Who knows what happened, and you will never know unless you ask. It may have been something like stepping on the reins or catching them on something and frightening herself and/or bruising her mouth or like kevinshorses said, chipping a tooth. I'm sure you have already ruled out teeth issues, but thought I'd mention just in case, you never know.
Also, sorry for chatting away too much on your thread!