Am i in the right to be a little upset at my girlfriend?

alright, so we hung out for like 8 hours today, i got there and she was acting different, she was also texting someone who i knew but she just wouldnt tell me who she was texting, she normally does but not this time, she knows i like to know who she is texting and understands that, but she then told our other friend who later came that she never texted this guy back, but i know she was, like alll day that is i talked to her about it and i am disappointed actually, is this situation alright for me to be mad?