Yes try a fluid change first. If your D light is flashing you have a trans dtc this would be an electrical problem, solenoid problem or hard failure of the trans. You will have to pull the code to find out more. What gear? do some research on stall testing A/T transmissions this is easy and will help you find out more. Search button top right.

Sound like a typical clutch pack flair up to me AKA clutch pack slipping.