Talk to Pediatrician on Constipation/ Refusing Milk

My baby boy is 0.2 months old and he has constipation. He usually poops after 4 days and till few days back he was feeding fine. He used to have 120 ml per feed. But now suddenly from last 3-4 days he's lost his appetite. He takes only 60 ml after 4 hours and goes back to sleep, he's sleeping more than usual. What should I do? He is not crying for milk and is not showing any hunger cues.