My DS, 13, has PE for last class at school. A good few times on the way home from school after PE, he will say he feels unwell. Feels shaky, drained, hot and cold. Then lies on the sofa as soon as he gets home. Mumbles responses, very bad humour, extremely irritable, can't focus enough to do homework, etc. I make sure he has a drink and something like Nutella on bread. He can take all evening for him to nearly recover, or most likely he is tired, drained and not himself until the next morning.
He is fairly skinny but doesn't do much aerobic exercise during the week.
This might happen other days after school when there hasn't even been PE - vague 'I don't feel well all over' - lying on the sofa, not himself, can't do homework, eats very little then has a day off school next day and recovers by the afternoon. His usual self is bubbly, chatty and full of it.
I would be prone to low blood sugars myself if go walking without eating just before, but usually would recover in 10 mins or from them.