She thinks you are not happy with her and that you want to get together with your ex. She thinks by you not telling her that you were trying to hide it which makes her even more suspicious as to why you texted her. She is wondering what else you have been doing with your ex that she doesn't know about. Women can be very insecure. Reassure her that you do not want to get together with your ex, that you do not have any feelings for her and that you used to be friends so you wanted to see what was up and that you did not mean to hurt her.
Do you want to re-connect with your ex? That will be harder for you to explain to your girl.

First of all I dont understand why she was in your phone. Sounds like she doesnt trust you. You both should sit and talk about the expectations of your relationship before going forward