This can be so tremendously hard for some people to understand (I've encountered it countless times over the course of my life). My being quiet does not mean I have anything to hide. I just don't have anything I want to, or feel like showing you.
Here is a good MBTI test I found from a fellow pinner. Strong ENTP with traces of INTP, ENFP, and ENTJ. By traces I mean maybe I took it like 12 times to get different questions and the results were all similar.
This Is What The Brain Of Every Introvert Looks Like
Introverts tend to get their energy from within; so being with people is draining. After a day filled with people or activities, introverts tend to feel exhausted and empty. They just need quiet time to come back to themselves.