Light blue stain on underwear!!

M

Hi! I'm 29 weeks pregnant with my first baby. Lately I've been noticing light blue stain on my underwear. The stain appears at the exact same spot in all my undies. I wear only on 100% cotton undies and I don't wear blue jeans or any blue pyjamas.

Also I haven't changed my laundry products.This is giving me sleepless nights. Please comment if you have similar experience.