menstrual irregularities can occur due to many causes,
the possible causes for your problem are,
-anemia, hormonal disturbances, recent illness, weight changes etc leading to scanty flow during periods(hypomenorrhea
-in case of poly cystic disease also hypomenorrhea can be seen,
-in case of unprotected intercourse
during ovulatory period there is chance for implantation
so you better observe in the next cycle if the same thing repeats get investigated, if possibility of pregnancy
is there go for pregnancy test once.