Social Anxiety. Hmm. An interesting diagnosis from your friend. Defensive in my opinion.
Focusing on the trip for a moment: I picked up on a couple of possible problems. Your friend and you in close proximity for a week. That alone can bring out the worst in both, if you are very different. Your shyness, or reticence to engage her in conversation, seems to stem from your fear of her belittling you. Your silence may have been a passive aggressive way to deal with that fear. She reacted by diagnosing you with a nervous disorder. So all did not go well.
I am sorry you were so uncomfortable on the trip, which I assume was a vacation.
But, you did finally speak up and tell her how you felt. Good for you.
The larger issue is your shyness, which you say became apparent in middle school.
Shy people frequently wind up in a psychiatrist's office, complaining of problems like the ones you describe. A clinician will use the term social anxiety, a diagnosis, to advise therapy or medication to help you overcome it. I have the same difficulty, and when I was told I had 'the shy gene', refused to believe my doctor. I thought because I dressed flamboyantly, and was a free spirit in many ways, it couldn't be true...but it is.
It seems you are a shy person. That is not a bad thing at all. It is when you begin suffering from your shyness, withdrawing from people and activities that you want to do, does it become a problem. I recommend therapy for you, shy girl. Time to blossom!