Implantation Bleeding or Period?

I’m usually on time with my periods; maybe a day or two late here and there but almost never early. I got my “period” 4 days earlier than my date. Day 1 was just spotting. Day two it was a full blown period. Day three it bled for only a little in the morning. I’m on day 4 and there is barely any blood unless I wipe. I had taken a pregnancy test the day I got this “period” and it was negative. My periods always last at least 3 heavy days and go on for a good 6-7 days with enough bleeding that I need a tampon. But this time, I bled for a day and a half but it was heavy. Could this be implantation bleeding or is it definitely a period?