WTH!?

Okay, so the past few days the man that I love, the man that I live with, my world, has been very distant towards me. I started to get really upset tonight because he hasn’t made me feel like everything is okay between us. His response to me being upset was “if you’re so upset then leave” I was a little taken back by that. I responded with “ummm and go where?” His response was “to the man you’ve been sleeping with” ......whoa. Okay. I was very shocked and hurt by what he said. I don’t really understand why me cheating on him would cross his mind. I am as faithful as faithful gets. I love him to death. Can anyone give me anything to understand what might be going on? We would be together for a year next month (if he doesn’t leave me)