Black / Grey poop 12 month old? (Picture)

A❤️C

My soon just turned 1 year so just started drinking milk these last two weeks. He's probably had diarrhea off and on since he started drinking milk but I just thought it was getting used to it. Today he definitely had diarrhea all day today and then this right before bed. Freaked me out. My sister that's an R.N. said its ok to wait to talk to his doctor. I'm thinking of going to get the toddler formula tomorrow (he was on the sensitive one before) until I can talk to his doctor on Monday.

Anyone have experience with this?