Missed/low period after sex?

Jeanette • Hi :) ❤️ Mommy to two little boys ❤️ 2.22.16 ❤️ 8.31.17 ❤️ best miracles ever!!!! :) ❤️

My periods are usually on time and have no effect from sex or anything. I was supposed to get my period 7/4. We had sex 7/3 and he finished inside me. My period came but its just like barely hanging on and only showing when i wipe and its light and little streaks thats it. It should be heavy and full on. Idk why sex and cum would mess it up when it never has before.