Breastfeeding

This is my first baby, and decided to breastfeed. We're gping on almost three  weeks, i used to be able to pump at least five oz minimum from each breast. And together could make a full 8 oz bottle. This week had been less, now I'm getting 2 oz at a time. Is this normal? Sometimes my baby eats and then an hour or less acts hungry again. We've gotten better about making sure he stays awake, andgiving both breasts for him to eat from. Should i be concerned? Things i can do to keep milkcoming other   than drinking water?