I breastfed exclusively for and my daughter still was constipated. Not because she had hard stools - just the opposite, it was still runny, but she would hold it for almost a week and cry horribly when she finally did have a BM (a big runny one – sometimes twice in one day) and of course very gassy a couple of days before the big explosion.
My pediatrician at the time said it was normal as long as there was no blood in the stool and she wasn’t constipated since I was breastfeeding exclusively and when she did go, it was runny. She also advised giving her white grape juice and using the thermometer for stimulation - neither helped. She like the others that have responded gave me the same reasoning that BF babies don't go as much, absorb more, etc, etc - but sorry, I wasn't buying it because I knew when she did finally go - she strained and cried.
This went on for several weeks at that point I told my friend who is a pediatric surgeon my concerns and she knew immediately that my daughter had an anal fissure (tear), showed it to me and then prescribed ointment that would numb the area so that my daughter would go more regularly (smaller BMs) allowing the tears to eventually heal. She held the BM because it hurt every time she went and would re-open the tear. My friend said that anal fissures are very common in infants on up to young children but most pediatricians will not diagnose it unless prompted. Needless to say I changed pediatricians because telling me what my daughter was going through was normal and not checking or offering some sort of relief was not good enough for my daughter. Once I started applying the ointment, she starting going every other day on a regular basis.
If BMs are soft but infrequent and if she's straining then I would have her check for anal fissures.