3 week old hasn't pooped in 4 days

anna

My daughter is 3 weeks old and hasn't pooped since Saturday. She was strictly formula fed and than Saturday I started her on breastmilk (supplementing some formula still when needed) for the past almost 2 days she hasn't any any formula only breastmilk. But she still won't poop. I've tried inserting the tip of the themometer/ q-tip like my doc suggested but nothing. not sure what else to do?