Can a short period actually be implantation?

My periods are ALWAYS six days long. Never shorter, never longer. Always, always, always.

Except for this one. My period started around 9 pm on Thursday evening. I bled overnight, all day Friday, and most of the day today (Saturday) It started getting lighter around 7 pm, and I just took a tampon out that had absolutely no blood on it.

This is suuuuper weird for me! What could it be? I haven't done anything differently, and I'm not stressed out or anything like that.

(TMI menstrual description) It was relatively heavy amounts of blood, ranging in color from bright red to a dark red, and no major clumps or anything. But again, nothing really flagged out of the ordinary until I just plain stopped bleeding.

Has this happened to anyone before??