How can I prevent this from happening again?

I had to leave this morning to fix one of my clients computers. When I got back home I found my guinea pig lying on his side and he seemed out of it like he was swimming in and out of consciousness. I petted him to make sure he was ok and he didn't start pop corning or lick my hand and he felt hot to the touch. I drew him a cool bath and he finally came out it. I am sure it was heat stroke but that doesn't make sense because before I left I gave him fresh water and veggies and I always keep the room at about 70 ish degrees.

I've been keeping a close eye on him and he seems to be ok, he's playing with his favorite teddy bear and is fully alert. I am ready to rush him to the vet if I notice any sudden changes.

Please tell me my little angel will be ok. and if it wasn't heat stroke then what was it??