Dating a friends ex, am I in the wrong?

[Company9]

My casual friend "Annie" dated a guy a year ago for about 2 months. Just recently I met "Bobby" and we fell for each other. Before anything happened I told annie that bobby and I have hung out a couple times and if she felt awkward about that. She told me "No not at all, go for it, He's only Bobby to me!" I asked her if she was sure, because I didn't want her to not want me to date him if she really did have feelings for him because her friendship meant more to me. Annie assured me that it was okay and not a big deal at all. Bobby and I have started to get more serious and now she hates me! She says I betrayed her and that Bobby is only going to hurt me.(he cheated on her) She says that I chose him over her and that I have no loyalties to my friends, and basically I am a crappy person. Am I in the wrong here? Is she just crazy? Any advice?