Help! 6 week old baby crying and screaming during and after bottle

hi guys! 

My 6 week old baby has been screaming crying more a less during every feed. He was pooing around 8times a day runny and had mucas in them he is on aptamil first milk. The doctor has changed his milk to lactose free and given us gaviscon this was on Tuesday and it hasn't help he hasn't pooed as much but did a hard poo this morning but still cries at every feed and looks in pain I really don't know what it is or what to do. any help would be grateful! thanks you so much