Formula and breastfeeding

I breastfeed most of the time but he eats so much I have supplement because my milk supply isn't enough but my son is soooo constipated.....he may poop once a day it's very watery and he pushes so hard on top of that he has really bad gas he screams in pain!!!...I gave him some gas drops which relives most of his pain but I'm really concerned he's only 18 days old and I feel lik such a bad mommy please help!!!