My daughter, once she reached about 2 months in age, only pooped about once a week. This continued until she started solid food at around one year old. Once she was eating food she evened out to once a day, and it has been that way ever since (she is five).
I was the same way, only pooping about once a week as an infant until solids were introduced.
I really do believe this was her normal cycle. She has shown no sensitivities to foods once she started solids, and I highly doubt it was something in my breast milk that was causing her to poop only once a week. She has never been constipated, and even when she was on her "once a week" cycle she never strained or pushed hard when trying to poop, never had those rock hard stools that can cause pain and bleeding. She has never tried to retain her stool ( I have seen kids that try to "hold it"), she just went when she was ready. She just had normal stools once a week, and not as much as you would think after not going for a whole week. My midwife said she probably just absorbed the nutrients extremely well and didn't have that much in the way of "leftovers" to push out.
The only time she ever really pooped "off schedule" was when we went up in an airplane. Then, no matter what day it was, she would go right about when we reached cruising elevation. This happened about 6 times. Weird!
I think that children's bodies all work differently. It is normal for some babies to poop all the time, but it is also fine that some wait. I personally was grateful for her poop schedule: it made cloth diapering a breeze!