Caroline,
It could be a lot of things I suppose, but most of us will develop changes in the mucous membranes of the nasal passages as we age that are not allergy (immune) related, plus we have structural changes as well. These would act as irritants.

You could check into Non-allergic Rhinitis if you feel it's not allergy related. Just make sure that you have no blood in the mucus.

I remember that you used to take Ibuprofen daily but that may have been only a temporary treatment ages back. Ibuprofen is listed as a potential cause of non-allergic rhinitis on some sites. I personally wouldn't take Ibuprofen as well as grape seed extract.

If you live in air-conditioning, just make sure filters are clean and unit is regularly serviced.

We can become allergic if exposed long enough. Maybe a neighbour has planted some shrubs or trees that affect you.

Lots of potential causes, but if you don't think it's allergy related then I would certainly suggest checking out non-allergic rhinitis information.