2) you handled yourself extremely poorly. you let an emotional response dictate your logical response. learn from that.
3) some people (i learned this the hard way, too, with my dorm roommate my freshman year) have this particular insecurity: they want to make it known to others that they are more "in the loop" than you are. this involves spreading people's secrets. if you're an information hub, people look up to you.
answer? don't tell her things you want kept secret.
here's what i've been doing for a long time: in my tightest circle of friends, i'll tell one friend one secret, then the next friend the next secret, etc, if i think any of my secrets won't fuck my life up were they to get out into the open (certain things, everyone eventually learns to keep to themselves). that way, when person B knows the secret i told person A, i know i can't tell any more secrets to person A.
~ dan ~