I'm so confused...

Your best friend is probably angered that you have a new friend and that she is not the center of attention like she once was. I wouldn't take offense to it. Girls do this all of the time. They say one thing, mean another, and then drama starts.
I think it is great that you are branching out and making new friends!!!
What I would do about you and your best friend: sit her down and talk to her. Tell her that you took her advice and made new friends. Also, set her straight and tell her that you aren't making anything up. Any person who questions you like that is not cool.......if she doesn't stop doing that, I would drop her like a box of hot rocks. That isn't a thing that a "best friend" would or should do. Tell her that if she can't be happy for you making new friends, then maybe she shouldn't be your friend.
I hope this helps! :) Best of luck!