Oh dear. I hope this is just precautionary.

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The fact that Haakon drove his wife to the hospital and she was able to get out of the car by herself is somewhat reassuring. I would have been worried if she been taken to the hospital per ambulance.
It "could" be a combination of stress, extreme fatigue, not eating properly and dehydration. That only a far out guess.

The Norwegian court made a statement prior to King Haakon taking the Oath yesterday. They stated that Queen Mette-Marit's absence was due to complications following the lung transplant she underwent in June. It added that such complications can occur during the recovery process after a major procedure and that the Queen would need to remain under medical observation throughout the day.