Did I go too far by breaking up with my almost 5 yr girlfriend?

I started a new job in which we had training for 6 weeks. I got in with 3 other people in which we started to become friends. We shared our cell phones and texted every blue moon. I texted this girl sam maybe 10-20 times over a 3-4 day period just to say whats up etc..My girlfriend got on our att bill because she knew i had these *new* friends. She seen a number she didn't recognize so she called it with her number blocked. She found out it was this sam. She confronted me about it in which she was p***ed off. I immediately lost my trust in her and broke uo with her. We have been togather almost 5 years.

Do you think we should have though it out first or is what I did ok?