How long has he/she been under there?
That's not long! Maybe he's asleep! :wave:
Will he come out if you shake his food. Try to chase him out to get an idea if he is OK or it is just heat related.
Will he eat and has he pooped?
If he is not himself within half an hour then I would take him to the emergency vet.
ETA - how has he been acting strangely
Strange in what way? Has he been eating? Have you checked his bum lately to check it's clean? :wave: