My friend is being mean for no reason?

I am very frustrated right now one of my friends is being very off and on with me lately its almost like literally one minutes shes nice the next shes not. I am really confused and honestly don't know what to do. So heres basically what happened like 2 weeks ago she broke up with her boyfriend that was using her I have been there for her and tried to give her advice she askes things like. Why did he cheat on me? I say well I honestly don't know you could ask him? Shes like no I don't want anything to do with him I want to forget about with. I say yes I think its best. Then out of no where she says what are you hiding from me? I say what do you mean? She says your hiding something what? I said we were just talking now your accusing me of something? She says im sad and accusing everyone of something now how do you feel about my ex I don't want you talking to him and I am not trusting you until I get proof your not talking to him! I said how do I get proof? Shes like that's all on you! I am just so frustrated with her because all I ever do for her is try to help her through this break up and shes being a snot to me for no reason what do you guys think I should do? Thank you for your help sorry its so long