Possible pregnancy. NEED ADVICE.

Rebecca • My name is Rebecca! I`m married and 21 years old! Jesus is everything! And Texas is the greatest state out there!!
So I am currently 2 weeks late and took a digital test about 4 days ago when I was only 10 days late and it was negative. I'm now 15 days late and have had no sign of AF (fingers crossed!) but why would it be negative I just don't know what else it could be? Ive been faithful with my BC pills since I was 16! (I'm 20) anyone have any ideas??