What Causes Vaginal Bleeding After Having Sexual Intercourse?

hello, i have this problem: to begin,my baby born in cesarian last december 31,2010.its our first baby .after a month we make sex with my husband as all is already ok i think...inside my vigina and my cesarian wound bcoz the blood already gone. so we try to make love after making love i saw pink color stain in my panty as blood but not yet red the next day its real blood flows and increasing after 3 to 4 days and more and continue...can you expain me why this happen?it didnt happen this thing before everytime we make loved. this happen only after having cesarian birth.the first this happen we went to hospital to check what happen but the doctor said its my normal monthly blood arrived so no need to worry,then i use now a anti baby and it says i will not making blood while taking this pill,a few weeks later we make love again and happen the same, blood cames out after making love first pink color but takes a few days in 3 to 4 days to real blood flows im worried about it:could you help me o explain why this happen?
thank you very much and im waiting for your reply:
its me,
NELLY