I would suspect implantation bleeding if it occurs a bit earlier than expected, if it were scanty, spotty, pinkish and not red and heavy like a period, and does not follow the normal pattern of a period (light to heavy to light).

The BBT should remain elevated and not decline to preovulatory levels.

There is often the feeling that it is a period with light cramps and bloating, but the bleeding is shorter, lighter and just not normal.

If a woman is usually regular, and now all of a sudden develops this type of early spotting, and of course is "at risk" for a pregnancy, I would strongly suspect conception.