Hey I Have Missed My Period By 15 Days Now

Hey I have missed my period by 15 days now which I usually never miss but I have taken a total of seven pregnancy test and they all come back not pregnant and they were clear blue digital and first responds I'm not stressed of depressed but I do smoke and I have had this lightning pain in my stomach light cramping at night feeling hungry more so often but if I was pregnant I would be around 3 weeks and 5 days why wouldn't that show up on the best test out there