What does a 1 day period mean?

I was 5 days late then had a 1 day period. Was too heavy to be implantation bleed I think. Took a test to have peace of mind and it was negative. What could it be?

(TTC, wasn't <a href="https://play.google.com/store/apps/details?id=com.glow.android">tracking ovulation</a> this month).

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