My Baby Is Only 12 Days Old And It Seems

My baby is only 12 days old and it seems as if my milk supply is decreasing. I can tell first thing in the morning that my milk is in, but during the day I'm constantly nursing the baby because he's acting like he's still hungry actually starving. I've tried pumping and pumping and feeding and feeding but nothing seems to work and when I do pump I get hardly nothing! Please help!