During fusion sex position i always have pelvic cramps after sex. can the cramps be due to constant hitting of the cervix?

Pelvic pain: "Pelvic pain after sex" may be due to deep penile penetration and a posterior flexed uterus, which may stretch the round ligaments. A change in positions which does not cause this kind of discomfort may be sufficient, but if the pain/cramping is independent of the position, see your gyn provider for an exam and pelvic ultrasound.