While her teeth were just done? It does not mean that she may not have more teeth problems.
They could have cracked a tooth, adjusted the bite so that she is hurting with the bit, or she could even have cracked the bars in mouth too, at that time, or since then.
Also could have a twig caught in there too, or piece of hay, or thorn?
Saw one with twig stuck straight up into palate.
Could have something else going on, but could also be the metal? Are you coating with anything?
Can you get a picture of what she looks like in bit?
Could also be browband is too small from side to side? Or too close to eyes?
Horses make me a better person.