this is totally normal for an infant who is breastfed. sometimes they use it all up. breastmilk is perfect for the human infant, and so the body can and sometimes does use up every single bit of it with no waste. this means your infant wont poop every day. this is a good thing!
just keep nursing. breastmilk is NOT constipating, and besides, constipation is not the lack of stool, it is hard, painful to pass stool. she is obviously not having that. ;) so dont worry! just be glad you are breastfeeding, and that your child is getting the absolute most perfect and best food for her body, and that her body is so perfectly and adequatly using it up that you dont have a messy diaper to clean. how can this be bad? LOL.
NO PRUNE JUICE. NO NO NO. she is NOT constipated. no reason to add that extra needless calories. i think they are telling you that only to give you a "cure" and there is no cure for your infant using up the most perfect food for her body. ;)
if shes gassy, its likely something you are eating is buggin her. try reducing or eliminating dairy, a likely first cause. second are things that would make you gassy, like broccoli, green peppers, or beans or something like that. just eat as healthiful as you can.
otherwise get some mylecon drops or something, pump her legs during gassy times (even pussing them up to her stomach to push the gas out!), and make sure to burp her well.
otherwise be grateful. no stool means no mess! LOL