Do 6 years olds do this?

Hey moms so i have a 6 yr old daughter and i don't know if its a phase or what but ever since she turned 6 she's become very bratty and i mean like i have a mini 16 yr old. She's very well behaved and is a sweet girl but man she has a attitude! Its mainly she has this habit of saying STOP with such an attitude if we ask her to do something more than once and its starting to become a problem. I do talk to her and put her in time out but idk whats been up with her, i feel like a horrible mother maybe im doing something wrong or is it that i have my hands full with her 5 month old baby brother but no matter what i will still have time for her even if im overly tired i still go outside and play with her or color but please help ladies!