Could you be. Just for fun :p
Shorts?
You change me once, You'll find me on your phone or next to it.
A clumsy room mate throws shorts wherever he likes after removing it or changing it -even on phone or sometimes next to it
You change me twice, You'll find me on the sea or behind it.
There can be many scenarios - I'll leave up to reader's imagination :p
You don't change me, because you trust me.
Obviously we trust shorts that it will do its job of covering (and won't torn etc) :p
You can change me, because I trust you.
But yes you change also
They said, those two things make me exist.
Shorts has two openings to easily insert your legs inside-two legs make it exist. It is made up of cloth and elastic. :p
Title:
My roommate's short riddle.
It is short - see shorts
Conversation:
rm: Hi, good morning! I have a nice riddle for you.
Morning is a time to change shorts
me: seriously? Is this not too early?
Describes some prefer to not change shorts so early in the morning
rm: c'mon, just guess who am I :)
c'mon, is the short form of come on