in my limited experience of broaching the poly topic with my mother, it seemed like it was much more about her ideas and pre-conceived notions and programming than it was about what was actually happening.
That is to say that perhaps you simply touched on some deeply rooted programming in your mother that sent her off the deep end. For example, if you've been taught since you were newly born that to have an additional partner is a sin against God, then your son telling you he's going to do that consciously publicly and honestly could make you "lose it"
99% of the people in the world with multiple relationships just do it by lying, concealing, and cheating. By denying the reality of it, they save public face and dont have to deal with the consequences of lying to their main partner. Just do it behind their back, and dont let anyone know about it, especially your parents. I think thats because most people dont have the character and communication skills necessary to do it "right"
So it's too bad it didn't go well telling your mother, but it's not unexpected. Parents are an older generation, and thoughts and language and concepts change over time. A key element here is to believe in yourself enough that her freaking out doesn't make you second guess yourself and your choices.