Hey Kenza,
I see the issue the third time a re-loaded your page. I’m not yet sure why this is happening though but please first try testing for a plugin conflict because it looks like there’s conflict with MailChimp JS in your site. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If that does not help, try adding this code in X > Theme Options > CSS.
.fb_iframe_widget_fluid_desktop iframe {
width:100% !important;
}
Thanks.