D&c 10 weeks ago for blighted ovum at 12 weeks pg, bled for 4 days after then stopped but bleeding every time after orgasm now, why is this happening?

Still healing? Orgasm induces vigorous contraction of Fallopian tubes, uterus, cervix, and vagina, which may lead to spotting blood from incompletely healing of mucosa from D&C. So, it's still logical to be patient and see as long as the spotting is slowly but surely becoming lighter and lighter. If spotting still lingers beyond 4-5 months after D&C, see your Gyn-doc for reassessment and counseling. Best wish...