Constipation in Newborn

Hi! So I'm a first time mom with a 3 week old who just won't poop.. We have her on similac soy and just switched to nutramigen today and she hasn't pooped for 24 hours and has pooped twice in the past 4 days. She also isn't eating much and what she does eat a majority of it gets spit up.. What can I do for a constipated newborn? I've tried a bath, rectal thermometer, bicycle kicks near the stomach. She's been crying nonstop when she's awake and nothing seems to help.