Milk Supply

My baby is 12 days old. She was in the nicu for the first week after she was born because she was 4 weeks early. So I had to pump. We’re continuing to bottle feed her because we need to make sure she’s eating enough. But Up until yesterday I was pumping 3 oz on both sides and it’s dropped to 2 oz and now it’s at 1.5 oz. Any advice how to get it back up? I would really like to keep her on breast milk. And yes I do drink plenty of water.