What Causes Delayed Periods While UPT Is Negative?

hi, i'm 26 years old. i had missed my period. today it was 77days. but two times i taken the HCG test. but the result was -ve. i don't have any symptoms.i don't know how many days start from baby's heart beat. i feel some beats in my stomach is it true. if am i pregnant then when i have take the home pregnancy test, or any other chance.