Sounds pretty close to what I felt, although from the other side of course. I was fairly happy as a little boy. I played sports, and sang the "blow it away with a shotgun" songs with the other boy scouts, and all the pictures of me are pretty much of a happy young boy. There were some signs, such as how I always had girls as best friends, and socially I acted much more like a girl, but I definitely did not notice them at the time, and definitely did not identify as a girl, and didn't have pretty much any stereotypically-feminine interests like dresses or dolls or pink.
I also started feeling gender dysphoria around age 12.
And if you ask me, part of it is middle school. I think a lot of us are more androgynous, and as such we really don't stand out as much as kids, since kids by nature are more androgynous, and really they tend to not even understand gender aside from its most obvious physical concepts, such as boys having short hair and girls having long hair, and boys wearing certain clothes and girls wearing certain clothes, and boys and girls playing with certain toys, but there is a lot more overlap at that young age, because kids pretty much don't care about societal pressure and how they're "supposed" to act, they just do what comes naturally. But then, in middle school, suddenly guys have to be uber-masculine, and girls have to be uber-feminine, and if you deviate from those norms suddenly it's a big deal. You get picked on for it, possibly shunned socially, and really start to feel out of place. Not to mention that this is when puberty starts, and suddenly the physical realities of the two sexes REALLY split apart fast, getting very much gender-specific, while as kids boys and girls are almost completely physically identical except for the genital area.
Anyway, that's my take on it, as someone who also had a very happy childhood, but then at about age 12/13 everything just started falling down around me.