I had a d&c 8 weeks ago, why have noticed "mild twinge" while in seated and upright position?

Hard to say. After any surgery there is a variable healing time. For you, depending upon many factors such as how far along you were, your individual anatomy, previous conditions such as pelvic infections, infllammation of the abdomen, scarring and fluid in the abdomen, you may need more time to heal. Signs of complications may include fevers, drainage or discharge, bleeding and you need to see your doctor.