Period or implantation bleeding?

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I started spotting on the day my period was to start, then stopped. It's supposed to be day 3 of my period and it hasn't come back. I usually have a full 7 day period with the 2nd, 3rd and 4th day fairly heavy. This has not happened to me before. My periods are usually very regular as well.. I posted in a different category, I think it was the wrong one. Any insight?