New MOM

My baby is a month and three days old. He is on Pro Sobee formula because he was spitting up to much on the other and his stomach would stay hard for hours and he would just scream. Now that he is on the new formula his stomach still gets hard after he eats and he still screams. Granted its not as often but I am so scared that something is wrong with his stomach. I mean he is healthy he weighed 8 pounds 11 ounces when I had him and now he weighs ten pounds. Can someone help me

Valerie