No period, Negative test

Hey everyone! So my period is 5 days late, I tested yesterday and it was a big fat negative! I am still breastfeeding my 9 month old, but I’ve had three regular periods since her birth that come on the 7th of every month every single time!

My husband and I had unprotected sex 1 time at the end of my period about a month ago. Other than that we have been using condoms! So I really don’t think I’m pregnant, I’m just confused as to why my periods had become so regular and now it’s vanished again.

Has this happened to anyone?? Any thoughts? Suggestions?