There are several possible explanations for thin, watery period blood.
My Period Blood is Light Pink and Watery - Be Bodywise
A guide to vaginal discharge colors: What do they mean?
What Does The Color Of My Period Mean? - Axia Women's Health
What is Your Period Blood Telling You? - Natracare
Vaginal Discharge Types, Causes, Symptoms, Treatment, Prevention
What is Your Period Blood Telling You About Your Health? — Freya
Is it normal to have pale pink and watery period blood? - Quora
What Does The Color Of My Period Mean? - Axia Women's Health
Is Watery Period Blood a Sign of Pregnancy?
Ruby Cup Explains: Why is My Menstrual Blood Slimy?