Late period but negative pregnancy test 🤔

Karen

So I had nexplanon in, took it out on July 28 to start trying to conceive. Spotted a little after taking it out then nothing until August 16 when I though I had my period but it was really just 3 days of light flow almost spotting but a bit heavier. So I was supposed to get my period on September 13 so is now 5 days late. And I got a negative pregnancy test. Could something else be the cause of a missed period?