What could cause light brown spotting throughout the day one week before my period is due, then nothing, if not implantation?

Not implantation: That was not implantation bleeding as it is usually a spot or two in mid cycle.

Many possibilities: This could be caused by many things, including ovulation, miscarriage, infection, irregularity due to stress or illness, and maybe also implantation. If your period does not start in a week or 2, or if you get continued bleeding or pain, see a doctor. Good luck!

Irregular period: It could be an irregular period or spotting. It is not necessarily implantation bleeding, although that is a possibility. Time will tell. A pregnancy test if you miss your period could tell, also. I wish you the best!