Is it normal to shut down when a "stranger" joins the conversation?
For example, I was having a conversation with a nice girl (a friend om mine) while we were waiting for the train to leave. Then a friend/colleage of her asks her if he can sit with us and she said yes.
Now I completely shut down and can't say anything. they start talking and all I can do is look out the window and listen to what they are saying.
Then the two of us get out of the train and I start talking again. her dad was picking us up but once we where in the car I shut down again because I don't know him.
This of course isn't the only time that happend. It happens everywhere, with everyone I talk to.
Is it normal and how do I get over it?