It appears to me that your camera is broken. Either your flash or Auto ISO isn't working properly. If it was, the photo at ISO 100 and f1.8 would be exposed properly even if the Auto ISO over ride was working. Take another photo. Turn Auto ISO off, set ISO to 100 and your lens to f1.8 and use flash. If the exposure is now correct then what was done before was a fault.
Nikonians®, NikoScope® and NikoniansAcademy™ are trademarks owned by Nikonians.org.
Nikon®, Nikonos® and Nikkor® are registered trademarks of Nikon Corporation.