I think my fianc� is cheating on me, what do I do? :(?

We've been together for 8 months and now live together. After we got engaged he said he used to look at other womens **** when we went out together. Of course, this made me feel quite sad because my are quite small (B) then he said he used to imagine being in a relationship with other women he'd see. Ever since he realised how depressed this made me he said he had stopped. However, a few weeks ago a girl named Jess started to text and call him, ending her texts with a kiss. Just the other night she called him and I was in the room, when I got closer to him he hung up on her. Now she keeps texting him. He said he doesn't like her and she's a slag. Am I wrong to be paranoid? What do you think? I mean he always goes out to his friends house for over 5 hours. I feel like i'm just a crazy woman, but I love him :(