3x lower conversion with same traffic. How to diagnose?

My woocommerce store has got 3x LOWER conversion rates starting with october. Traffic and all the other metrics on Google Analytics have stayed the same (comparison with other months then only +-25% chance).

Traffic sources and quality has stayed same
Bounce rate
Page views
Funnel behavior
.... - Did not find any unusual actions from basic Google Analytics

What things to check and how to diagnose the problem?

Is your product seasonal?
I guess you already thought about that as being a reason, but I just wanted to throw it out there anyway