Taking pills can have different effect to our body, it can cause delay of menstruation, few or large amount of menstruation or sometimes like your case only spotting. Ngdedepende din po kasi un sa ating monthly cycle or kung meron tayong prob sa ovaries natin like PCOS. Minsan naman pills is depende din sa nagtatake if hiyang b un s kanya or hindi kaya better is to consult your OB to make sure what's the cause and what best pills to use. But spotting is normal if yun lang ung naproduce mo na fertiled eggcell.
spotting kasi can mean a lot of things if ever observe and keep your ob updated