1 Week Old Poop

I am having some baby poop questions. I've been breastfeeding since day one. Once my milk came in, she started having that seed mustard yellow poop as described. Between yesterday and today, she is still having them, but they are super runny with a little solid poop with it. My MIL says not to worry, but I still wanted to ask. Please let me know if anyone else has experienced this!