What Causes Cramps And Spotting In The Mid Of Menstruation?

i have been trying to get pregnant for 6 months now, i had sex before and during my ovulation and still not pregnant. After making love during my ovulation, 8 days after i felt some slight cramps and saw small drops of blood when i washed myself, i felt it was implantation bleeding but my menstruation started on the 13th day. Am getting desperate, please help me