I don't know what the problem is but I wish I did because I have similar things going on. I have a chronic sinus condition and recently had an abscess in the sinus cavity. It was treated with heavy antibiotics but I was told it would probably come back because my sinuses do not drain properly and, like you, I never have any nasal congestion because, quite simply, sinus fluids do not make it down my nose! Everything just sits there in a pool, fermenting into abscesses!

I have had the cameras up the nose and down the throat but they do not show sinus problems - you need an x-ray for that, which was how the abscess was picked up.
I am awaiting assessment for an operation now to sort out my sinus condition but I have not heard great reports of success.
Ask for an x-ray and/or a scan to see what is happening. Incidentally, it is unlikely to be a tumour as that is pretty rare.