Caught my fiance in a pickle over a phone.when confronted she pretended like she knew it was me any suggestion?

i am overseas i got the texting app for my ipod i remembered when i was home there was a guy texting her calling her y and stuff.i didnt think anything of it bc i trusted her and she seemed like she was disgusted this guy was doing it. but i decided to play a joke and pretend i was him..was going to end it right away but she kept the convo going up to the point where there was plans made for that weekend then when i asked her about it she pretented like she knew it was me the whole time..i know i should believe her but it just doesnt seem right. what do you think?