Advice ladies please?

I haven’t been able to keep up with my daughters needs with pumping, she’s in the NICU and I can’t put her to breast because we are monitoring her feeds so I’ve decided to switch to formula... we’ve been doing like half n half or some here some there but last feeding she puked about 10ml of formula and had brown watery poo this morning.... it’s going back to normal baby poop looking, but what’s everyone experience been with a transition from breast to formula? Just want to make sure it’s normal for some pukes as she has bowel problems and I’m hoping that’s not the case but just a reaction to changing over, since formula is a little harder on the bellies.