What Caused Very Light Period With Dark Red Blood After Unprotected Sex?

I had unprotected sex with my boyfriend about 9 times last month, this month I got my period and the blood was really dark red and brown. My period is extremely light, I was able to wear a panty liner my 2nd day of my period which I can never do because my period is always heavy my 2nd. My period usually last 4-5 days and lasted only about 3 days. I don t know what s going on.