How is stress related to an early period?

I've been kind of depressed and stress for these past months. However, I mask it all under a smile to everyone. I tend to act extremely happy if I'm sad or stressed out.

Suddenly I just got my period now, a whole week earlier than usual. How does this relate. My sadness and stress is one thing (emotional state) meanwhile having periods is a bodily fuction, physical state.

How is stress related to an early period?
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