Why does my 3 week old's poop suddenly look like this?

I exclusively breastfeed, my baby eats often, is gaining more than enough weight & has plenty of wet & dirty diapers. He has always had the mustard yellow "healthy breastfed" poop, but his last 2 diapers have had this green, lettuce-like stuff in it! Other than this, I have also been having some shooting pains in my breasts the last week or so. I thought maybe it was thrush but it kind of comes & goes. Baby has been spitting up & choking a lot too. I told our pediatrician & he said it's fine as long as he's gaining weight & still has wet diapers. But it still worries me. He doesn't spit up after every feeding or even every day, but when he does it is a LOT of spit up. Any idea what's going on?