anxious mummy

Baby is 1 month and 3 days. Initially baby was on similac. Since he had trouble pooping and had phelgm, I changed his milk to karihome goats milk. Everything was ok at the beginning once I changed the FM but recently he started to cry whenever he wants to poop. Stool still soft. He farts alot. Wad mistake am I doing here ? And he also has nappy rash. Wad can I apply besides desitin? Pls help me??