still no AF

Amanda • My husband & I have been married for 3 years. We have 1 son who is 2 & currently pregnant with twins!! A boy & a girl!
AF was suppose to be here the 21st. I have done 2 pregnancy tests since and still BFN. I'm very fatigued, moody, irritable, & I pee more than normal. I never miss my period nor am I ever late!! The only time I was, was with my son & I took a digital 2 days passed AF was due & got a positive. Wonder why I'm still getting BFN this time???? When should I test again???