First time late period!

kaci
I'm 21 in a 3 yr relationship, have used the withdrawal form of "BC" since we got together. I track my period on here and I'm never late. I'm usually synced up to the moon phase up until last period. 
I'm 4 days late and nothing has changed in my sex life. I took 2 tests both which came back negative. I have no symptoms of my period or any pregnancy symptoms for that matter and I always track my CM so I always watch for implantation bleeding. I'm. So. Confused. Anyone ever experienced this and ended up being pregnant or not pregnant? What could be delaying my period? I don't take any medicine, no stresses in my life, idk. Leave your thoughts please!