Baby's poop & breast milk

My daughter is a month old, she is both fed with formula and breast milk. However, I have noticed that when she drink Brest milk her poop is runny and she goes quite often. For example she will drink breast milk and then she will have 3-4 soiled pampers in a row, within the hour. I was wondering if anyone else's baby has this issue? Or if any one knows if it's normal? I know to some degree it is but is is supposed to be still going on even though she is already a month old? when wil she get used to the breast milk?  Or is it the combination of breast milk and formula that make her have these type of stools? 
She has a one month check up next week but at her newborn visit her doctor said it was normal for breast fed babies to have runny poop, but I'm curious if other newborns have this issue? Especially formula & breast milk fed babies. 
Sorry for the long post and repetition of questions!