Thanks for the query.
In case of menstrual cycle with around 21 days duration that can be ovulatory bleeding.
It can be because of lesions like cervical erosion, polyps etc.
fluctuation can cause menstrual irregularities like irregular shedding etc which can cause inter menstrual spotting.
If you are on contraceptive
measures that could be breakthrough bleeding
So if the bleeding occurs again better to get examined once.