Don't worry...this is totally normal. I am also breastfeeding and I have a 4 month old baby we was pooping regularly for the first 3 months of her life. She all of a sudden decided that she was going to poop every 7-8 days. My doctor told me that if she doesn't poop for 16 days, then contact him. So it must be something normal for babies around this age to go through. Don't give him/her anything to try and get them to pass a bowel movement. It's just their digestive system still getting used to the outside world. As long as he/she doesn't seem like they are in pain, I would just let them be. They will eventually pass something...and it's usually really big (smile).
Good luck and be patient!