All illnesses and stress on your body can increase blood sugars.
Unfortunately, high blood sugars can also make your illness and stress worse so it can feel as if it spirals.
I don't think you have to tell your GP (or anyone ) you have COVID. But I want to thank you for protecting others and staying away from work when you are unwell, especially when you don't get paid for three days (which I think is shocking and ultimately bad for your employer if it encourages people to come in a nd spread sickness).