I had the worst flu I've ever had in December too. Lasted from 18 Dec - 27 Dec. It was crazy because I'd just completed a 4 week powerlifting strength gain cycle in gym, and PR'd in all my lifts, so theoretically was the strongest Ive ever been. And yet during this flu, I could barely go for a 10 min walk before feeling exhausted.
Started with a dry cough, quickly became like I had glass in my throat and I was dreading each time I coughed. Lasted 3 days. Then came a crazy fever and the deep flu sleep that comes with that. That lasted 3 days. Then I had these crazy intense abdominal pains, like I'd been punched just below the rib cage. Half way through this saga, I felt vaguely suicidal for 3 days, especially at sleep time, when I was questioning the purpose of life itself.
My Mum also had it a few days before me, I almost certainly contracted it from her. She had very similar symptoms but not quite as bad and recovered more quickly. Neither of us went to the doctor, we just assumed it was seasonal flu and toughed it out using OTC medicines.
The Convid19 symptoms sound exactly the same.