You have trust issues. You looked and got numbers out of her phone that you didn't recognize so you could call those numbers and see who she was talking too because you don't trust her. Whoever you called must have gotten your number from their caller id and found out who you were and told her that you called them. You told her the truth because you knew you were caught and lying about it would have just have made it worse.
It's really hard to tell if you feel so bad about it because of what you did or because you got caught and you feel bad because now she knows and trusts you even less and you may lose her or you have already lost her because of it.
A person's trust in you is a hard thing to regain once it has been lost. I wish I could tell you there was a simple solution, but there isn't. She may never trust you again and then there is no hope of getting back together. If she gives you the chance to earn that trust back , you better be sure you can give it 100% or don't even try. She will test that trust and you better pass the tests.
You have probably been lied to or cheated on before and the trust you had for someone else was lost and it has caused you to not be able to trust someone else for the fear of being hurt again. The same has probably happened to her before as well. That happens to lots of people and it's not easy to overcome.
Trusting someone means being vulnerable to them and opening yourself up to the chance of being hurt by them, but it's the only way to truly love someone and have them love you back.
Tell her you want to trust her more than anything , but you trusted someone before and they betrayed that trust and it was devastating to you and made you feel so foolish that it's hard to not fear that happening again.
Tell her you don't mean to do it. You don't want to feel like that and you feel bad for the things you have done to her because of being hurt by someone before.
Tell her you love her more than anything and if she will give you the chance to have her trust you again that you will give your heart to her and trust that she will never break it.
You better mean it too!!!