Why do nsaids (diclofenac) cause heavier periods?

Increases bleeding: Nsaids can slightly thin the blood causing more bleeding and bruising.

However...: Nsaids can also help decrease menstrual bleeding if started early before period begins.It does this by interrupting chemicals called prostaglandins which makes the uterus cramp & bleed. Studies show blood flow is decreased 20-50% in some women who use nsaids. If nsaids are begun once a period is underway already, prostaglandins are not blocked and heavier flow can happen (as it has for you?) instead.