Thanks for the query.
If you had unprotected intercourse
around the time of ovulation then there is possibility for pregnancy
Usually menstrual blood can change the color during periods.
Dark blood usually indicates collected blood which is slowly coming out from the uterus.
And the quantity of the menstrual blood also can alter from cycle to cycle depending on many factors like hormonal
So no need to worry, most possibly this could be your regular menstrual period only.
If you still have doubt, go for pregnancy test once which will rule out the possibility.