Seems to me that if a child is feeling so poorly that they need to lay down and cannot do any work at all that it warrants a call home to the parents to ask about potentially going home/pain relief etc etc. Of course we don't know what instructions you've given the school in the past about such things.
I would double check with your child (and with whoever was with him during break if you can) whether he hit his head during break, or whether he ate/consumed anything weird during break. Break being the perfect time, of course, for kiddos to have an accident and bump their heads/fall etc. I mention this because the one time I can remember at school feeling so sick and grotty that I needed to lay down and couldn't focus on work at all was after I hit my head during break time and had concussion. I didn't want to tell anyone what happened in case I got into trouble (we were playing on the play equipment we weren't supposed to be anywhere near at that time, so I was scared I'd be in trouble).
Another potential worry (though your child is 8 so maybe not - but stranger things have happened) is the potential that they were prompted to try a vape or some other grotty thing during break time - a young child trying something like that (they might not even know that's what it is, sadly) could easily display the kind of sudden horrible sick/need to lay down symptoms you've described. Particularly if they were feeling absolutely fine when they went to school.
I would also pay attention to anything that involves a headache that is so bad the child needs to lay down/cannot focus on work: it may be worth a trip to the GP and asking about things like migraine or perhaps things like having his eyes tested etc to make sure there's nothing else going on that would make him poorly like that all of a sudden. Better safe than sorry even though the chances are very very small that it's anything worth a big worry.
It may be worth a very kind and non-accusatory chat with your child just to see if you can figure out any other information about what might have prompted them to feel poorly.
Whatever the cause, I do think the school should have informed you that your little lad was poorly to the extent he wanted to - and had to - lay down, and couldn't focus on his school work. I certainly would want to know. I'd convey that via a written email etc - not necessarily a complaint, but a note for the future that you expect to be told if your child is poorly like that again.