Pregnant before missed period

12 days before my estimated period I wasn't feeling good and for the hell of it took a home pregnancy test which quickly came up positive. Went to the doctor which confirmed with another urine test that I'm 4 weeks pregnant. I'm so nervous as I'm approaching my estimated period start date that my period is going to come and ruin this. Anyone have any advice? Anyone else find out they are pregnant even before a missed period.