3.5 week old - 48 hours without a poop

My baby will be 4 weeks old tomorrow. He was pooping between 4-6 times a day consistently, until it suddenly decreased to 1 poop a day for two days, and now he's gone 48 hours without a poop.

He's breastfed only and is still peeing regularly. When he does poop, it looks normal. He's been eating more than ever (cluster feeding, I think) for the past 4 days or so, and spits up after every feeding (but nothing excessive). He seems slightly fussier than normal, but nothing alarming.

I'm a first time mom, so of course I'm a nervous wreck over everything. Has anyone else's baby suddenly decreased their poops around this age?