Has my girlfriend cheated on me or not?

Me and my girlfriend are really good at the moment. But despite that I think she has cheated on me. The reason for me to think this is because sometime in November we had a huge argument and a guy called kwame had something to do with it. She then told me kwame is a room mate to her friend. And then later on said kwame has always been her classmate. I don't know how we stopped arguing about that in November. So early this year she went out clubbing and we argued after she went out. Only a couple days ago I took her to a hotel for our anniversary and kwame texted. And she deleted the message. I took her old phone and it seems that when she went out clubbing she meet up with him when they were out. After they meet she texted him a lot of kisses. When I asked why she said he's irrelevant. And said he only wants to speak her when he wants to speak that's why she deleted his message. And when they were out, they only spoke the whole time. Which I don't believe.

So please help me & tell me what you think. If you think she cheated on me, how can I get her to confess it?