Panting and shivering

[RumCustard]

My dog was panting earlier, went out for walk, did a couple of loops around the field and he was fine doing his normal sniffing and running after scent. Got home and a few hours later he was shivering. He sat by back door and went outside and lay down. he's moved off his bed onto the cold wooden floor, walked outside, done a long wee, moved across lawn and sat down. I'm going to take him to the vet first thing but has anyone got any ideas?

Bit late now as you've probably seen the vet, but it can be a sign of pain. Hope he's okay.

@Lucisky thank you for responding. Yes seen the vet after a dreadful night! He wouldn't settle and I feared for the worst. Anyway vet said dysplasia and arthritis prob because of the change in weather. Old bones!!