Goldfish lyin down at the bottom of the tank?

I have had y blac moor for over a week now. He hada small case of fin rot but is now alost completly healed. Today i came in the roo and he was laying at the bottom of his tan but he was moving his fins alittle and breathing. I don't now whats wrong! This morning he was fine and swimmin all around and now hes just laying there! Is he going to die?He didn't have any disease other than the fin rot whici was alost cured.