Why does my dog whine like if its in serios pain?

Today my 2 year old Chihuahua dog was just fine,but then all of a sudden he just layed down,seemed very sad and shaked alot .my dad picked him up to trie to see what was wrong but he just whined and seemed in alot of pain.I tought it was Gastric Dilatation-Volvulus but it said its rare in small dogs and mostly when its young.can you please tell me whats wrong with him?