I Think Shes Cheating

I am sat here alone and really need to share what just happened.i am 18 years old and i have been dating my girlfriend for 2 years now,i thought we have had a pretty healthy relationship until now.just over the week ago i was at her place and her phone rang and i was in a position to see that it was a guys names who i do not know,she ran to the phone and stopped it ringing and said it was Daniel from work ,i knew for a fact it wasnt but i let it go.she went to visit her father for the week and i picked her up from the airport today,so we got a bite to eat and while we were eating she got a text message from the same guy and i saw it and it was asking her when she will be home,so i obviously couldnt hold back  anymore and confronted her and she said he is just a friend ,now i look back her bahaviour has been different but she has got a lot of issues with her family and she does have a lot on her mind and she says this guy is just someone else to talk to,i just dont know if i can believe her as she has tried to hide this from me but,she says she hasnt cheated and wants to be with me ,i just dont know what to do .