relationships

i just started talking to a boy that my ex bestfriend "fell in love with" but yet she moved on so quick after they ended, i feel wrong considering i was once her bestfriend, but i like him a lot. i don't want her to get mad and spread a lot of drama about me because she's a bit immature and we're in highschool so things tend to get believed easily and you get hated on for every little thing you do, so i really don't know how to take on this situation, and it's eating me up right now.