I’m so confused and lost

Ive been on nexplanon birth control for almost 2 years now, and have had a very regular periods with it. Anyways! I’m now 3 days late, I haven’t had sex in 40 days (tomorrow). I took a test 2 days ago to ease my mind and it was negative. I have absolutely no sign of my period coming except for sore breasts which is never a sign of my period. I’ve only ever had sore breasts when I was pregnant for my son (I think that’s why pregnancy is in the back of my mind)

A pregnancy test should by now 100% show up positive if I were to be the .01% to get pregnant on that, correct?!

Since my periods have always been super regular on nexplanon my bleeding wouldn’t change after two years right?!

Someone helpppp. What’s going on with my body