My ex is a dick to me, and he is really nice to my parents?

Like he is really rude and just sais a lot of stuff to me. Then when my mom or dad comes around, its like bam he is nice as hell. I dont get it at all. What would you think if someone was really nice to your parents but when they left they started being mean to you. We tease eachother but sometimes he is REALLY mean. What the ****?