Give it time honestly. My mother is the only person I came out to who didn't react well initially, she thought it was just an attempt to sleep around and that it was unfair to my now-fiance. She believed even though she barely knew him at the time that he was okay-ing it just to placate me and secretly hated it and so on. She also didn't believe I understood what love really was because I wanted that.
I gave a long winded rant about what I believed love was which at least got her to stop trying to convince me I was too immature to really be in love. She then saw my relationship with my now-fiance grow and saw how in love we were. Once I became serious with another partner about a year later, I mentioned her in casual conversation as my partner, and my mother was very surprised, she had thought because my relationship with my fiance was so strong, that I had dropped the whole poly nonsense. It was a big eye opener for her to see that we could have a very happy functioning poly relationship. She then met my girlfriend and absolutely adored her. Since then, while she doesn't personally agree with my lifestyle, she accepts it fully and is very happy to hear about and meet new partners. My whole poly family is invited down for holidays and my parents have never been negative about my lifestyle since or asked me to hide it from their friends or the rest of our family. I believe what really made the difference for her was seeing with her own eyes that it worked, and how truly happy and in love we all were. Once she saw that and it blew her assumptions out of the water, she was happy to accept it as a legitimate and ethical lifestyle.
So don't worry, its normal to have negative reactions initially. Be honest, without pushing your lifestyle on them. As your mother sees over time that it isn't going to go away, then she may be more likely to ask questions and try to understand. Also, she may never really want to know details, but hopefully when she sees you are happy and have healthy functioning relationships, she will at least become more accepting and supportive of your choices.