What is going on?!?!

My 6 year old has been purposely doing things to annoy me.. Like repeatedly pushing her hands in my face. Kicking me in the stomach. Kicking me in the face, running off on me. Taking her sisters toys and laughing. then laughing when I yell at her (or slapped her butt when she kicked me in the stomach) Has been a big sister since she was 2, so is this a jealousy thing?? She's not the baby. I don't get this! But it is making me so mad! I don't know what to do anymore! She has always been defiant with me. But it's gotten way out of hand lately. I'm bout to fucking cry!