Baby not pooping

LT

Two days ago she didn’t poop until her last meal of the day. Yesterday she didn’t have one poop. She’s a formula fed baby that is usually pretty regular. She hasn’t been eating quite as much the last few days either, so maybe that’s it? How long can a formula baby go without pooping before you worry? I’m also travelling by plane in a few days, hence why I am so worried about it.