To answer this question, it is definitely something else. You're not a bad friend. Your friend seems to be having her own issues to react the way she did. You apologized and tried to fix things, but the way she reacted is because of her own issues, it's not because of you. Sorry you're having a tough time with this. Personally, I think you should just let her go. You don't need her friendship if she treats people this way. Hang out with the other friends from the group that you mentioned.