My Period End At 4/24, And I Had Sex With

My period end at 4/24, and I had sex with my boyfriend at the same day. After 15 days, there was some light brown liquid come out from my vagina. Then two days later, I had my period again. However, the blood was less then normal. Then three days later there was no blood, but there was kind of black liquid come out from my vagina, and it have been two days. I am wondering what happened to me. Is it really serious? Thank you!