Paddyanne. So sorry to hear you are suffering and for so long. I admire you for soldiering on. I think if you had flu you would have been forced to take a day or two off anyway. Coming from a Farming background, I know that there are those who cannot have a day off. The animals need feeding Christmas day and everyday no matter how ill you feel.
sea water
I use Sterimar. It's a cleanser and needs regular use and the effects are noticeable over time. It might well be sea water but it is purified so clean! I too am prone to sinusitis and don't want to use antibiotics so just soldier on trying to keep my sinuses clear and rinsed out.
I agree with the point that this could be an allergy as it seems to have taken up residence. I'd be inclined to tell people that it is!
As for what others think - ignore them! If you had any kind of flu you would be past the symptoms weeks ago. If people make you feel uncomfortable then wear a bell round your neck and to their questions about your foreign travel say "Not been anywhere really, just had to pop across to Wuhan for some items to do with the job a couple of weeks ago, but no holidays or anything."
*In general*: I'm fed up wit the COVID-19 hysteria! It spreads like seasonal flu, is not as bad to experience as most flus and has a similar death rate. Seasonal flu kills people all the time but nobody reports it or gets in a panic. For example, the US Centres for Disease Control, (CDC), estimates 30,000 people died between October 2019 and February 2020 of seasonal flu. Did you see the News Headlines? No. Me neither. It's normal.
Just behave sensibly, wash your hands well. If you are in a very high risk group (e.g. have pulmonary disease, or very weak immunity) and COVID-19 is among us in a big way, avoid crowds. If you are unfortunate enough to get flu or become ill and cannot manage, phone 111 for advice. But please do not start to get anxious now! It's a waste of time being worried and upset about something we probably can cope with all too easily.