If you have missed your period since August 31st and received a negative pregnancy test result on September 19th, it is recommended to wait at least one week before retesting for more accurate results. However, if you are still experiencing symptoms such as back pain, fever in the morning and afternoon, and continuous white discharge, it is advisable to consult with a healthcare professional to rule out any other potential causes. They can provide guidance on when to retest or recommend further tests if necessary.