No other seller here is going to know that. Did you setup a promotion code for free shipping? Did you change your shipping settings lately? In your Shipping settings did you check the box for ‘Free Economy’ shipping? Have you looked at the product page and the order details to see if there is something there to give you an indication? Do you have a different buyer account or a friend who can check and see if it shows free shipping when you add the item to your cart?
Other than this, your question should be directed to Amazon to investigate. Click the ‘Contact Seller Support’ link at the bottom of this page to ask Amazon if you are unable to determine the cause yourself.