What Causes Bleeding After Intercourse?

Hi there, i am 23 years old wit a 5yr old daughter..... on 8th march i suffered a miscarrage at approx 8 wks..... since then i had a period which finished about 5 - 8 days ago however after havin sex with my partner i have started slight bleedin again?? I had some sort of infection after the miscarrage and was given metronidazole 400mg to take for a week which i did and i finished the course... i was checked for any sti's and the result came bak negative, do u have any ideas as to why i am bleedin again??