At her age I had cropped hair and lived in jeans and sweatshirts. I was a total tomboy and loved Lego. However, if I was mistaken for a bit I'd be really indignant about it.
My DS went through being upset about being a boy a few years ago at the sane age. He said he wanted ro be a girl which was quite shocking to hear.
I researched things thoroughly as he'd never shown signs off disliking his body he seemed as enamoured of his parts as most boys and one of the first ways they rule out whether a child is truly trans or not is by working out if there is a perceived social benefit to being the opposite sex. So not just that they would feel happier that way. The next time my son said he wanted to be a girl I asked him why. At first he just said he didn't know but I probed until he finally said that the girls at school didn't get in trouble as much as when they were told off they just cried and got let off. He then elaborated that if he was a girl it would be ok for him to like dolls and cooking and art and all the things he likes and he wouldn't be laughed at. So it was totally a perceived benefit issue.
I managed to reassure him that there were plenty of male artists, his uncle (BIL) is a SAHD and that most of the top chefs are men. I also suggested not getting into trouble as much at school
His confidence has grown massively and he's now planning on being a chef when he's older and is very vocal about gender equality at school. He's got very good at shutting down other boys saying the girls can't play football or girls saying boys are rubbish at dance etc. Since I told him it was ok to just like what he likes he's not once said he is or wants to be anything other than a boy.
Don't jump straight to your DD being trans OP. She's had 2-3 years of school now with some kids who might be growing up with very rigid gender roles. DS felt all wrong amongst his peers until I told him to just like what he likes and suggested maybe some of the others were just trying to fit in. Lo and behold, as soon as DS declined to play football in order to make a bug hotel, several other boys couldn't wait to bin off 'boring' football as well.
Give your DD room to express herself and tell her it's ok. Be a quietly supportive influence. She will appreciate and respond to it.