Period 6 days late..all negative tests.

Hey there! My husband and I have been trying to conceive for 2 years now. I was on the birth control shot for a little over a year and went off of it in September of 2016. It took quite a while for my periods to return and this past year was a roller coaster. Early last year I had very irregular cycles, but since August of 2018 my cycles have been between 36-38 days. My last period was on December 12th of 2018 and I still have not started a period. I’ve had cramping and slight nausea, but no bleeding. I am now technically 6 days late and all of the pregnancy tests I’ve taken have been negative. Just wondering if any of you ladies have an insight. Thank you.