Is my girlfriend cheating on me ?

My girlfriend and I have the same phone and I accidently picked her phone up thinking it was mine and I've had a hunch that something had been wrong over the last few weeks so I looked at her txt MSG from one of her friends and it said " I love u and Jim ( that's me ) together but I'm thinking this will bring u and your sister closer" my gf and her sister just got back from Vegas 3 days prior to the MSG and she also asked "have u gotten any txt" and my gf wrote " he was real busy with work" and that did not apply to me cuz my gf and I were at a destination wedding and at the time of the tzt I was golfing with the wedding party And then a few hours went by and her friend txt " any txt lately" and my gf wrote " he prob won't txt me here that's a good thing". then her friend wrote "is Jim crabby" and she replied " no he is really really good" So when we got home a few days later I had a talk with her about how I was stressed ou about the weddding expenses and had not been my usual self for about a month and I said I noticed she had changed a little and point blank asked did you hook up in Vegas ? or are you talking to a gut what's goin on if anything . She said no I was with my sister Bla Bla Bla.. and it was about a half hour talk So I felt better. But my curiosity got the best of me and a few days later I checked her phone again and noticed the tzt MSG about he didn't txt cuz work was busy and the one after it were erased. Now what should I do??? Do I have another talk with her?? Please help thanks