Yules - not sure if it's the same or not, but Pie used to go ages between poos from a very young age (like from about a week old she only went every 7 - 10 days). When she would do them she was obviously in discomfort, she would cry & cry until it was over. She definitely wasn't constipated, they were normal consistency... I had her checked by the GP , LC etc & all said she seemed otherwise fine & to basically wait it out. Seemed like the length of time bewteen poos was making her uncomfortable, LC suggested the sensation of it starting to move was unpleasant?
We tried Brauers, Infant's Friend, massage (clockwise around the abdomen) but the only thing that really made a difference was when she started solids just before 6 months & all of a sudden she was going every day or two.
Not sure if this is the same as your situation, but thought I'd put it out there.