I agree if it was implant bleeding it would be just a day or two of very light spotting. Most people don't even get implant bleeding. It is normal for AF to vary in flow and length on occassion.
Pretty sure it was your period. If it was just implantation bleeding, usually that's only spotting for a few days. Having your period be a day shorter or a day longer isn't that uncommon. If you still feel you may be pregnant, test in a few days. Hope this helps.