Me And My Husband Intimate Last 16 Of Aug..without Any

Me and my husband intimate last 16 of Aug..without any precaution.. But I checked next day by urine test...there was a negative report...and after that we had sex 2 times...with precautions.. And my periods date is 2 of every month...I was shocked becoz my periods on 2 of sep was not come....and my periods cycle is never late....it should be on time always...but I was checked by urine test again..on 4 of September..... And the repot shows that I m pregnant..... How is possible....plz help me.....and suggest what should I do.....now????