Green poop

Hello! My 2 week old has been having dark green poop and we don’t know why? She is almost exclusively breastfed. She only gets a little formula if it’s the middle of the night and I’m too tired to feed from the boob/she’s not satisfied after the boob. But it’s been pretty rare that I do that—especially as we’ve gotten the hang of things.

Her poop was the yellow mustardy seedy that it was supposed to be and then changed to this dark green.

She’s feeding well. Typically spends 10-20 per breast—if she spends a long time on one, she won’t always take the second though. I rotate which breast she starts on. And she seems to be having enough wet/dirty diapers.

My main concern is that she’s not gaining weight very quickly.

Any insights?