From what I understand.. Spotting can be caused by sudden estrogen drops like that which happens right before ovulation.. Some women have spotting when implantation occurs.. Some women have spotting right before AF... And some can have a little bit of spotting b/c of something that happened to their cervix, like somehow irritating the cervix after DTD or scratching the cervix while checking CP.
Now women who have patches of spotting, long and/or irregular cy's and/or spotting which replaces a real AF.. And are TTC, should visit the Dr to make sure there isn't a problem such as PCOS (just an example).
Sorry I couldn't be more specific.. But I HTH anyway