So my baby is 3 weeks old and a cardiac baby following his open heart surgery at 14 hours old. Due to being on lots of medication after surgery he started on low dose feeds of cow and gate first infant milk. Going from pump fed to taking a bottle by time of leaving hospital. He left hospital on two medications one being furosemide once a day 0.3ml and ranitidine three times a day 0.23ml. And 70ml feed me every three hours.
Couple of days out of hospital he was guzzling milk like he's never been fed but as it was going in it was coming out just as fast via sick and poos whilst gurgling all the way through. We new with him having lost weight from surgery that, that was no good and. It healthy. So we changed his milk to aptamil reflux and regurgitation- all being well as it stopped the sick and no more gurgling and his wind was coming out fine (little windy bum) 
lol he now has gone constipated struggling poo, to the point he cries out in pain and looks red faced due to straining but still pooing 1-2 times a day.
What do you fellow parents advise, I change his milk again so he not constipated or is there something I can do to help his bum and tum cope ??