please give any knowledge you have

So, my son was born at 34 weeks and was completely healthy, in the hospital he was eating liquid ready made formula, once I got home I had to switch to powder and within a DAY it made my son so constipated he would scream when he pushed and his poop was balls, so I switched back to liquid and within 2 days his poop was back to green/yellow runny newborn poop. The only problem is he still screams and cries and is in so much pain when he poops but his poop is back to normal and has been for 2-3 days now.