DON'T PANIC! My daughter went through the same problem in the early months of her life. She only pooped once every 5 - 7 days. This went on until we added solids to her diet in the 5th month. This phenomenon is completely normal for breastfed babies. It doesn't happen to most of them, but it is TOTALLY normal.
A few things you can do: give her water, as much as she would take, as often as you can think about it, don't force her if she is not interested even a few drops is better than nothing. this will not interfere with her breastfeeding.
Massage her belly, always in clock-worth direction, push ightly with your fingers as if you are walking them in a circle on her lower belly. you may even feel bubbles under your fingers as you do that. do it every night before she goes to bed and whenever you think about it during the day, when she is naked.
Another good exercise is to take her legs and fold her knees toward her chest/belly, do that 8-10 repetitions couple of times a day, ti helps things inside move in the right direction.
There are a few other movements you can do, if you are interested in these exercises you should look up "itsy bitsy yoga" they have classes and a book with all sorts of exercies you can do with your baby. most of them help with digestion. I'd like to think that my daughter benefited at least a little from them.
Another option (which worked for us) is if she has not gone after 5-6 days, and you feel that she may be unconfortable, take a Q-tip, dab it in a little baby oil and VERY GENTLY poke her in the butt. after a minute or so everything will come gushing out, so be ready.
I know this was a little long-winded, but I hope it's helpful.
Good luck to you!
Oh, and one more thing, I just read that somone else mentioned the parsley stick, I thought that my mom was the only crazy person to recommend this (as an old Bulgarian tradition), but apparently it's used by other. Glad to hear! We actually used the parsley on my daughter several times, and it worked like a charm!