Sorry to hear that you are suffering. I was stuck in A&E for over 8 hours prior to Christmas with what you are describing, and the majority of people there had viruses or were children suffering from Strep A. Ever since my diagnosis, any illness (which tends to be like clockwork around late November!) causes my BG to rise. My Libre looked like a rollercoaster prior to Christmas, with big post prandial spikes and generally higher than average levels. I am fairly certain this is the liver kicking out adrenaline and glucose to help the body ward off the virus. High BG can also have a negative effect upon the body's ability to ward off illness though. It took two and a half weeks for things to return to normal and my GP signed me off from work for 3 weeks.
Have you tested for Covid?
I am a teacher, so coughs and colds are par for the course, but like you, in the past I would fight off the illness in a few days. Twice in the past five years I have been struck down with viruses that have lasted around 3 weeks. Stress is something that can also affect BG levels considerably, so trying to focus upon your well being and not worrying about things that are out of your control (easier said than done I know) can help.
I hope you start feeling better soon!