Help! my goldfish keeps opening and closing his mouth non-stop?

I dont mean to break the bad news to you but i think hes going to die:(..usualluy when that happens(iv had goldfish before) they open their mouth to catch air bubbles floating around...because they dont have enough air. and if he swims aroung and then juust lies on his belly..it means that he doesnt have enough oxygen and is going to die. goldfish dont live that long htough so you can always get another one im sorry:(