Realise why this could be happening to you and why others ain't
having this problem? 
The answer to this mystery lies within the feel you give to people.
When people are on two extreme ends - the feel they give is very
different - something like a werido or wearing a mask kind of
feeling. To others, they will think that being quiet is the real
you and when you start babblering your conversation, people
impression will be like 'hey, Orange isn't that quiet afterall.'
But then the next time they were see you, etc - you kept very quiet
again - you will be sending weird signal to people who doesn't
really know you and they won't take chances to know the real you
based on impression - which is sadly true for people in this world.
Since you have small circle of friends, it is a natural tendancy to
spark a conversation with others when the chances arises. This urge
appeared to be most likely unnatural - when you wanna get to know
people, to give people the impression 'you-are-so-yourself' is
always the key to everything, because you will be sending the
I-am-being-natural vibes around. There is no mask - people will
accept you for who you are.
First and foremost you ought to work on is your self confidence.
Trust me, you have no communication problem whatsoever - the thing
you truely lack is self confidence in front of people. Understand
why at times you kept real quiet? Its because the either you can't
enter, or you don't know how to enter the conversation, therefore
the key is to keep quiet. (Or the times when you really feel like
being quiet - PMS, etc).
To be natural with others and yourself, is to exude self
confidence. Having self confidence when others are around, you
won't be on the two extreme - instead you will go with the flow and
things will get better.
Cheers
