A pattern that I observed in myself today

While reading the Boundaries book, and understanding better what causes certain behavioural patterns in us, I had two similar experiences, with the same pattern.

Basically, when it's with someone - who for one reason or another I feel nervous - i find it hard to make small chat with. He/she'll ask me, "How's your day?" or something like that. Then I'll reply - but nervous. Then after that.... hmm. I know the pattern, but don't know how to describe it properly.

But anyway, why do I do that?