Frieght Under GST

Sir, I Have Filed the GSTR-3B for the month of July, But once after filling the return i came to know that fright charges are also comes under GST. we are into Textile business and we have purchase from local and interstate for which we are paying the tax while receiving the goods.how should i take into account the same, as i submitted the returns is there any possibility to do the changes, or is there any penalties for the non payment of tax on freight under GST.