I'm 31 weeks pregnant my last period was aug 13 is there any way i could've conceived on september 4?

Yes: in a 28 day cycle, ovulation would have occurred on 8/27 but the fertile window includes a few days before and after that and your cycle could have been a little longer meaning you ovulates a few days later. Even if your pregnancy dating is based on an ultrasound they are plus or minus a few days ( more the further along you are at the time of the ultrasound). Hope this helps.