I had no idea I was trans as a kid. My mom tells me that I was more well-behaved, "nice," and much more verbal than other boys, and she could see it long before I ever came out as trans to her, but I wasn't consciously aware of any desire to be female back then.
And then all of a sudden puberty hit me like a ton of bricks and just like that I hated every single thing about being male, and almost immediately knew that I wanted to be a girl instead. Mostly it was physical... I hated how male sexuality worked. Erections felt wrong, hell, having a penis at all felt wrong, and was a CONSTANT annoyance. I hated the body hair, cried over and over and over again on so many nights after my beautiful unchanged soprano singing voice changed forever, but there was also social stuff, like hating machismo, hating being expected to be tough and act like a dudebro, hated that I couldn't seem to cry at anything even though I really wanted to, and hated the complete inability to do things that were cute, pretty, and feminine without endless social ridicule.
It got worse and worse with pretty much every year, as my body continued to get further and further away from the female self I wanted to be. I tried praying it away, I tried being a more feminine guy, but nothing worked, I needed to be female.
I still don't really understand how I could seemingly have no dysphoria as a kid, and then all of a sudden have my gender identity so abruptly shift to completely feminine in middle school, but it happened how it happened. Gender is a pretty confusing thing to understand sometimes.
So nowadays I just kind of see myself as one of those gender-nonconforming tomboy girls who suddenly grew out of it and got super-feminine at puberty. That's the only way I can seem to make sense of it.