3 weeks old

My 3 week old (premature) son slept through the night again. He has been doing this for a week now. However, this week he did not gain weight.... he is maintaining weight but not gaining....
Last night he did not pee in his diaper, he didn't pee until after he nursed. He usually has 6/7 wet diapers a day, I change him before every feeding. And during the day we eat every 3 hours. 
I stopped pumping after a week, could my supply have dropped to much to support him?
He's sleeping so I don't know.