What Causes Dark Brown Bleeding And Sharp Pain In Vagina?

hi doc. I had unprotected sex about 3wks ago and recently got my period but its not heavy n bright red like usual, its dark brown n only appears when i whipe... Also i am getting these sharp pains n my vagina and it feels like i have to pee, this happens about every few ins. Can u help me to kno wat mite cause this plz... thx