Black moods and toxic exes

Once per month, I have these black moods that descend onto me from seemingly nowhere. It doesn't matter what I do or don't do, what my thoughts are, or positive things I try to tell myself, all efforts are futile. I simply must wait until the cloud passes. It's extremely hard to deal with especially given all of the responsibilities that I have in my life.
So of course when my ex-husband came to pick up my son this afternoon, that only exacerbated my bad mood. I brought up some issues of concern to my ex which I should have known would be a waste of time. He will discuss nothing and takes no responsibility for anything either. Long story short, it turned into a verbal altercation where he ended up calling me some colorful names including a nasty cow and a fucking cunt. My 6 year old son was present for the entire exchange. He is such a sweet and sensitive little boy and the fact that he has witnessed all of this hate and poison makes my stomach turn. Obviously I can no longer communicate with my ex at all. Which means I must co-parent with no communication. How the hell can I do that? And I must put a plan into place to ensure that nothing like today ever happens again. I owe my son much more than that. I plan to pray about this extensively because I feel so helpless.