Breastfeeding

I gave birth a week ago and decided to breast feed. Everything is going well, but seems like my daughter is constantly wanting to nurse which is expected. The other day she got frustrated and I thought maybe she wasn't getting any milk, so I supplement with formula a few times that day.

Now I think she's trying to get constipated due to pooping pebble looking poop or semi formed poop that is kind of soft. What can I do to help the stools be kind of loose? She doesn't seem uncomfortable and today I haven't gave any formula just been nursing her even through the frustrations. How long will it take to get the formula out her system?