Has anyone ever gotten a BFN one day then it turns into BFP?

i took a test (the cheaper brand of ept) that said that it could tell if you are prego from day 1 of your missed period. i took one saturday and it was neg. i kept going back to check it hoping it might change but even after two days it didnt:( i went to the doc monday and had a bt done cause i couldn;t wait and it came back as a BFN:( but the doctor said it was probably too early to test.well today is thrusday and i took a test yesterday morning and it came back neg, but this morning when i checked it it says positive. i think i'm suppose to start any time between saturday and monday. i think the test says positive cause it sat over night so i plan on taking another one if i dont start by monday. so my question is has anyone else had this happen before (even tho i kno your not suppose to read results after 10 min) and if so were you really prego or not?