Aiyoh, 5 days no poop is a bit worrying lor, even though breastfed babies sometimes go longer between poops, best to call your pediatrician just to be safe and get their advice, especially since you're mixing with formula. One of my mum friends had the same thing happen and the doctor said to check if baby is still feeding well, gaining weight, and if the tummy is soft or bloated, those are the signs they look for. In the meantime make sure baby is drinking enough and not dehydrated, and some mummies swear by gentle tummy massage in a clockwise motion to help things along, but really, quick check with the doc gives you peace of mind!