Hello again, and thanks for your additional information.
There's no doubt that he's uncomfortable, or in pain, and may be itchy under his belly again. His tail being down may indicate pain in his bladder or intestinal tract, even though he urinated and defecated normally, today.
There is a condition called 'bloat', which can affect dogs; the stomach twists in on itself inside, and causes discomfort and pockets of gas to form in the abdomen. Usually, the stomach may appear bloated, but bloat can occur without an obviously bloated belly, also.
If you have an ER vet center in your city, I'd recommend an immediate visit. He will be evaluated and treated, to help him feel better. If you don't have an ER vet center in your area, look in your Yellow Pages under Veterinarians, and find listings with 'emergency hours/emergency phone' and call until you find someone to come into the office for you, or to make a housecall.
I hope he's feeling much better once he's treated by the vet!