Me and my friend were sitting on the bus, and so were my exgf and her friends. My friend & I were talking, and I said that's my ex he was talking about. So he asked my ex if we used to be bf/gf. She said no, and acted surprised about it. What can I conclude from this?

She does not want anyone to you you are her ex, she misunderstood the question, she is angry with you, who knows. How long has she been your ex? What difference does it make?

The fact that you still call her your ex, and the fact that you are bothered that she denied it says you're not over her. Let go man, all it does is hurt.