Chris DOh, what a day. I too will say a prayer for you.
As Kathryn said, our kids know better than anyone how to push our buttons.
I can remember saying some really awful things to my mother when I was growing up. Now I can see what a wonderful, wonderful mother she is and I truly appreciate and love her. I am sure your son will do the same (if not now, when he is older, 5 is so young!)
I think the reason I could say mean things to my mom was because I knew she would love me no matter what, so she was a "safe" person to vent on. In a mixed up sort of a way it is a compliment to what a great mother you are that your son can feel secure enough to lash out at you. He just has to get it out somewhere.
Given what a critical support you are to everyone on this board, and how you always know the right thing to say, I can't imagine you being anything but "the best mother in the whole world".
Be gentle with yourself!
Keep us posted on your son.