I am a psychologist and work with this age group a lot.
This is a tricky one. You describe your DD as having changed dramatically and overnight. This can be explained by adolescence. Many readers/ posters will have had similar experiences themselves, or with their children. It can also be a symptom of other things such as overwhelming stress and anxiety, abuse/bullying and/or specific illnesses/conditions.
At this age, assuming behavioural issues/symptoms attributable to adolescence at the expense of identifying true cause is common (diagnostic overshadowing). Less common, but does happen, diagnostic under shadowing can occur whereby adolescence is overlooked and typical teen behaviour is pathologised. l
We don't know what happened in the consultation with the GP. It could be that your DD was frustrated because the doctor was dismissive or failing to attend to the issues your daughter thinks are key. It could be that the doctor had some potential diagnoses to explore, requiring exploration beyond the information that your DD thinks is relevant, but is still important to explore from an informed medical perspective. Perhaps the doctor touched on, or picked up on, an issue that your DD is finding overwhelming and this caused her to flee. She may be telling herself that she walked out for other reasons as she is in denial, or she could be fully cognisant of an issue but cannot face disclosing it.
The important thing is not to make assumptions about any one explanation. I would listen to your gut. If you sense there is more to this than the usual teen issues, that's significant.
My suggestion is that you let your daughter know you are concerned about her well-being and you are there for her. You can, at the same time tell het that her behaviour was not OK. In the incident at the GP, you can say that there were ways of handling the situation that would have helped her, not worked against her. You can reinforce that it's OK to feel frustrated and angry, these feelings provide important information that something is wrong. We need to listen to them, understand what is causing them, and then take action to make things right for ourselves. Most of the time, the most productive way isn't to deploy the nuclear option. You can also say that doing this during the teen years is very hard as our brains are wired in a way that makes over-riding emotions with reason very difficult once emotions are heightened.
I hope this post doesn't cause you more worry. There may or may not be something concerning going on. My key message is to hold both in mind, try and ensure your daughter knows that even if she feels something is unspeakable, you can safely handle whatever she tells you, and in the meantime, you can place expectations or her re her behaviour.