day 3 of misses period

it's driving me crazy! my period is 3 days late which I'm excited about but the tests keep coming up negative I have symptoms stomach cramps unbelievable heartburn nipples and boobies sore. is it true sometimes the tests don't show up till 21 dpo? and if so I need to wait another week to test again or should I go to the doctor? thanks for all the advice this will be baby no 1 and I don't know what I'm doing lol