Hi, I fed & given formula to my 1.5 mth old baby. I gave her formula on night time or when I was away.?

Now, she didn't p motion for 3 days already. This is her 2nd time being like this (after several times of formula (Babelac) feeding in one day). I've asked the Pediatrician and she says it's normal for fed babies. I would like to know is this because of the formula or my milk that cause this. Would be appreciated if some tips/advice shared, especially on how to make her p motion. Thanks.