How old is he? Shivering does not sound good. It could be a reaction from spot on treatment, but how bad of a reaction is the question...can you take his temp.?
did he have a recent vaccine? what?
If Anubus is on here she may have better idea of symptoms and whether you should get to the vet asap.
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. - Unknown