What Causes Menstrual Blood Odor After Placenta Removal?

I am 40 and have had 2 babies in the natural way they are both 12 and 6 now. an hour after the birth of my last child part of the placenta was retained so I had 2 have it removed MANUALLY (I was put on antibiotics 2 prevent infection)and since then the menstrual blood every month became very smelly like rotting flesh but over the years it has been getting worse. Its that bad I almost gag. my cervical smears are normal