trying not to cry at work.

Long story short..my fiances ex has been causing a lot of fights with me and him. When we first started dating she messaged me on Facebook and said that they slept together the week before and that he wanted to be with her. I let it slide since he argued that it didn't happen and I really wasn't mad since we weren't officially together at that time. They have a 2 year old son together and yesterday he was at her house for an hour and 15 minutes. He swears on everything that nothing happened and he was there for a bit and went to the store. I don't really believe it. I love him so much and I want to believe he wouldn't do this to me. Since he had his son last night, we didn't talk about it because I'd hate for an argument to break out in front of him. I asked him to show me the receipts from the store and he told me "I'm not showing you shit".  And I cried myself to sleep. This morning I told him I was leaving for work early and he asked me to go to his other sons basketball practice. I said yes. A few hours later he says to me "I love you Mami". That's his nickname for me. I said "love you too" I still feel sick to my stomach and on the verge of crying. What would you ladies do if you were me? We live together and I really gave up a lot to be with him. Benefit of the doubt? Help!