Fertilization occurs but not implantion??

Shelia
Why is it that i will get a postive pregnancy test and then the day of my period or 2 to 3 days after ill start bleeding? What can cause our baby not to implant? Has anyone went through this. Im going to maternal fetal medicine on the 27th to make sure im all okay. If anyone had this preoblem can u let me know how and if its fixable