Hello, I'm hoping someone can shed some light on my (odd) situation.

A little background:
I've always (since 14) had very very regular periods...they come on the day expected, never ever late. 28 day cycles. like clockwork.

I am a 29 year old woman and my husband and i have been trying to conceive for about 6 months now. For the last 4-5 months , i've noticed i will have light spotting on the day that my period is due. Never a lot, not even to fill a pad. About 2-3 days later, I get a full period.
Is it possible that I am calculating wrong and my period is actually starting on that 2nd or 3rd day, or is the spotting actually counting as "day 1" of my period.
For some calculation efforts:
My last period was January 10th (when i started my last 'spotting') and i was due on my 'next' on Feb 8th, which was when i got my "spotting" again. It's now the 11th and i still have not gotten my 'full period' but am experiencing menstrul cramps.

I took a pregnancy test and it came back negative, and have no symptoms.
I'm wondering: Is my math off? Am i experiencing implantation that turns into a period? is that even a thing?
Or perhaps im calculating wrong and should be calculating from when my full period comes...not the spotting.

Any insight or experience advice would be greatly appreciated!