Missed period

ok ladies, I am 6 days late on my period. I’ve taken 4 pregnancy test 2 at 3 days late and 2 at 6 days late and all four negative.. what’s going on here.. this will be pregnancy #5 if I get pregnant. After I had my 4th daughter I never got a period up till 3 months ago and my ob had to prescribed rx to jump start my period after 1/12 of not having period, after baby was born three months ago. The last three months have been consistent but this month I’m 6 days late.. idk what’s going on. My last 4 pregnancy when I missed I was pregnant. What do you think?