Period is 5 days late

I have a regular 28 day cycle and have never missed a period before.
.
My period was due on Wednesday.
I had really light spotting for 3 days, 6 days before my period was due - which I suspected to be implantation bleeding.
Now, I'm 5 days late.
.
I haven't tested since Thursday, but the home pregnancy test results were negative!!
.
What in the world is going on?
Has anyone ever experienced this?
Should I test again?