Why commerce changes my header?

Well before blaming WC, first check if your theme is WC compatible. If not, follow this link here -> https://docs.woothemes.com/document/third-party-custom-theme-compatibility/

I tried it, I copied my page.php to woocommerce.php and I replaced the loop with the coomerce function. It doesnt work ?? Why?

Well looking at the code of your site, you can see there is a header background image that is applied inline on the subheader container. This leads me to believe it is dynamically adding that where on the product page, that inline code isn’t there. Perhaps that will give you a hint on where to look as my help is definitely limited as I can’t see any of your code.

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