I Got My IMPLANON (etonogestrel Implant) Out Of My Left

I got my IMPLANON (etonogestrel implant) out of my left arm on march 12th 2015. I have had it in my arm for 3 years after i gave birth to my now 3 year old son. My husband and I would like to have another baby and i started my period the day I got the ROD out of my arm and it lasted for 7 days. I was supposed to start my next period on the 9th of this month and that was 8 days ago. I took a 88 cents pregnancy test the day after my first missed day and it came up negative so i waited 3 more days, Still no period and took another 88 cent test from (Wal-mart) and it is STILL negative and today is April 16th 7:07 PM standard eastern time. USA. I will be 23 years old on the 22nd of this month and weight roughly about 280 lbs I am healthy other then my weight. I has also noticed signs of pregnancy but that may be because i have been SO SO SOOO stressed out about A LOT including having a new baby. So with all that said. I need advice. Why would i not be having a period STILL and why are the 88 cent tests from Wal-Mart turning up negative? Whats wrong with me?