Red Diaper Syndrome
Until recently I'd never heard of this syndrome. It's an infection that leads to babies' nappies (and in feeding mothers' breast pads and pumps) to turn pink.
Articles can be found here in Red Diaper Syndrome and in Breastfeeding Medicine.
If your baby has this I recommend you make an appointment with your GP and take the details from these articles too. The articles suggest the condition is rare, and while frightening to see this in your baby's nappy, it is not actually that serious.
Images taken from Breast Feeding Medicine. Copyright 2018, Mary Ann Liebert, Inc.