When he jumps up, does he look (or lick) at any particular place on his body?
If he does, it could be a pain he feels in that place.
Check his skin carefully, under the fur for anything you may not be able to see easily. There could be a thorn or sharp grass seed embedded that he only feels when he lies in a certain position. Or a Tick maybe?
To me, it sounds as if it could be a sudden sharp little pain he gets sometimes. If he continues to do that it might be wise to see the vet