What Causes Light Menstrual Blood Flow Post Unprotected Sex?

hello my periods comes pretty regulary and i recently had unprotected sex and for some reason this month my period came so weird like light the first day the second day was pretty light blood with very little clotting the third day was the same as the seond and then the forth da i wake and its no period at all can you help me inderstand when i my period comes regulary and quit heavely on the first four days