Similac Total comfort

My son was constipated previously with enfagrow, so i changed to similac TC 4 months ago. He was drinking well without any constipation issue. And in sudden, he refused to drink milk, always he only drinking like 60 to 90mls. Not more than this. Its going to be almost 2 weeks, Im worried about his daily intake. He is now 18 months old, any mommies experienced the same ? #firstbaby #pleasehelp #experiences #18monthold #similactummycare