Diarrhea looking poop should I be worried

Danielle
Last night my LO had reddish brownish runny poop. I called my pediatricians nurse hotline and they said it could be normal. He has now had really yellow runny poop 2 times today. We were supplementing with formula but he is now mainly on pumped breast milk. Could it be that he is only on breast milk? Is it diarrhea?