Somewhere around third or fourth grade, kids start noticing themselves the way other people might see them. A wrong answer said out loud, a shirt that's "babyish," a joke that didn't land: things that wouldn't have registered a year or two earlier suddenly sting. That's not your child getting more sensitive. It's a new piece of brain development coming online.
Self-consciousness is new, and it's not fully calibrated yet
The ability to imagine how you look from someone else's point of view develops steadily through elementary school. Around 8 to 11, kids get noticeably better at it, but the skill is new enough that it tends to overshoot: they assume everyone noticed the thing they're embarrassed about, when in reality most peers didn't register it at all.
The impulse-control gap is still real
The prefrontal cortex, the part of the brain responsible for impulse control, weighing consequences, and regulating emotion, is still years away from finishing. Neuroscientist Jay Giedd's longitudinal brain-imaging research found this development continues into the mid-twenties. At this age, your child has more self-awareness than a kindergartner but not yet the brakes to always act on what they know.
Why that combination looks like moodiness
A kid who's newly aware of how they come across, but not yet skilled at managing the feelings that awareness brings up, can look moody, prickly, or overly sensitive seemingly out of nowhere. Not a personality shift: two systems, self-awareness and self-regulation, simply developing on different timelines.
What actually helps
Naming the gap out loud takes some of the pressure off: "It makes sense that felt embarrassing. Most people probably didn't even notice, but the fact that you did is your brain doing something brand new." Your steady, low-key reaction to their bigger reactions is still doing more good than any explanation.