Based on an average 28 day cycle your dates of conception were Dec 5th-Dec 13th.
dic 9
If you are due on sept 1 and are 26 weeks pregnant then your conception date should have been on dec 9, cause i im 25 weeks pregnant and im due on sept 10 and my conception date was on dec 18
It looks to me like your ex might be the father. Have you told him that it is possibly his baby?