She purified herself following her menses then she saw some drops of blood

Women’s problems with their menses are endless, and one of
the causes of that is their use of contraceptive pills and pills used to
prevent menstruation. 

People never used to face such problems before. It is true
that there have always been some problems since the Prophet (peace and
blessings of Allaah be upon him) came and since women were created, but this
situation where people are so confused is something regrettable. However,
the basic principle is that if a woman becomes pure (her period ends) and
she sees the tuhr (white discharge) which signals that her period has indeed
ended, then any brownish or yellowish discharge that she sees after that, or
any drops or wetness, is not menses, so it does not prevent her form
praying, fasting or having intercourse with her husband, because it is not
menses. 

Umm ‘Atiyyah (may Allaah be pleased with her) said: “We used
not to regard the yellowish or brownish discharge after the tuhr as anything
important.” Narrated by al-Bukhaari. Abu Dawood added the phrase “after the
tuhr”. Its isnaad is saheeh. 

Based on this we say: Everything that happens after the
definite tuhr does not affect the woman and it does not prevent her from
praying, fasting and having intercourse with her husband. But she should not
be in a hurry to see the tuhr because some women, when the flow becomes
lighter, hasten to do ghusl before seeing the tuhr. Hence the women of the
Sahaabah used to send pieces of cotton to the Mother of the Believers
‘Aa’ishah on which there were traces of yellow, and she would say to them:
“Do not hasten until you see the white discharge.”