My 4 yr old was the same the last 2 weeks of school. She'd sleep fine at night and would wake in a chipper mood but as the day went on she'd get clingy and be upset for no reason. Wasn't in pain or have any symptoms of illness, couldn't explain what was wrong. Late afternoon, she'd get lethargic and peaky looking then develop a temperature, refuse dinner and have to go to bed early. It was like that on repeat. She's alright now as if nothing was wrong.
Growing? 2 of my dcs used to go pale, tired, unwell looking for a few weeks, then suddenly perk up and I'd find they'd grown. Ds did this last month, had grown an inch. He also used to be like that when constipated