Flipping the Switch

So about a week ago I went to stay at my boyfriend's flat which he shares with a few friends. I usually avoid staying the night there as I spend the whole time I'm there on edge and worrying about what his fiends think of me. This time wasn't any different. When I first got there it was only him in, so I wasn't too anxious. But then as soon as his flatmates got there I immediately went into quiet mode. It's almost as if someone flips a switch in my brain that turns me into a completely different person. If someone asks me a question I give the shortest answer possible even though I know I can sometimes come off as rude and should really say more. I try to think of things to say if someone does ask me something but when it comes round to it my mind goes completely blank and all I can manage is a few pathetic, mumbled words. The change happens so suddenly it's almost impossible to control. Often the rational part of my brain knows that there is no reason for me to fear the situation, yet my body doesn't cooperate and goes into anxious mode anyway.