ANSWER:
If the spotting comes before the flow of blood of menses, this is not part of it. You should pray and fast as usual.
But if it comes during the menses or after it seeing your purity, it is part of menses, unless it exceeds 15 days.
You must disregard any spottings that come in before and after the period after seeing your purity. It is not your menstruation until you see the flow of blood and you must not stop praying, fasting and you can have intimacy with your husband during these spottings. You must wash the affected areas, change the underwear and renew your wudu before paying.