I saw my bf with another girl🙁💔

Me and my boyfriend have been together for a year, almost two. He's always been a good guy with me. But recently he has been lying to me. So yesterday I went to the mall with my aunt and the girls. At the moment he was at work in the mall. He came out at 5. I texted him saying "hey I'm gonna be in the mall around 5, so we could hang out after you come out." So I received no text. I texted him 3 times and no response. It said delivered. So I'm walking towards forever 21 and I see him going down the stairs with a girl. And I called his name out loud. And they both came over to me. I introduced myself to her "hi I'm melvins gf." Then I told him "so you didn't receive my text?" Then he said no. And I said "who's this"? And he said "this is Catherine I was with her because she wanted to buy a jacket and she wanted me to come with her." What do you guys think about that?