Babys Poop

So I posted my concern about my son eating A LOT more frequently now & not pooping as much. He is breastfed but I started worrying I was losing my supply. He usually poops about twice a day but lately hes been going a couple days. A few days ago I gave him a bottle of formula & the day after he pooped! It was a little harder and had more of a green tint to it than what he normally poops. Well now its going on the third day again he hasn't pooped. My fiance said he pooped yesterday while I was resting but he said his poop was black!?! I'm kind of freaking out. My son is 17 weeks old & he is starting to teethe a little bit.. I just need to put my mind at ease. Is this normal at all?