I think It is "난 뭐야?", which means " what am I doing?". It is often used in a confusing/incomprehensible situation. It is a casual form which is used between friends or close people.
I give you some examples to help you understand it.
1. Your friend asked you to be a drummer for his band. So You said "yes" and you went to his practice room. But there was already another drummer practicing with other players. At this moment, you can say "난 뭐야/뭐니?" to your friend.
2.You have a boyfriend. But he didn't invite you to his birthday party. And you found that he had traveled with another girl. In that case, you can say " 난 뭐야/뭐니?" to your boyfriend.