thanks for using health care magic.
The first thing to consider after missing a period in a sexually active female is pregnancy
The bleeding you experienced for a day sounds like implantational bleeding which can occur
within the third to the fourth week after last menses and not on a fixed
and defined day as you thought.
However, there is need to do a confirmatory test.
Do a home pregnancy test with early morning urine and if negative, re do in a weeks time, if still negative pregnancy would must have been ruled out.
Best regards and hope this helps you.