Baby squirming and pressing when he poops

My LO is 10 weeks old and fully breastfed. Husband and I noticed that he squirms, presses until his face turns red and screams a lot everytime he poops from when he was a newborn. Once he manages to do the deed, sometimes after two hours, the poo is always mushy or runny and never hard so we know he is not constipated. Why does he do this? It looks like he is in pain and has a hard time doing the deed. Does anyone know what is wrong? His paediatrician told me to do the qtip up the bum and that hasn't helped at all.