You could try testing for allegies.

I use Olbas Oil (which contains peppermint oil and menthol) which is excellent for sinus problems and other headaches, and is available over the counter in the UK. It's better than those prescription jobs that you squirt up your nose, which can actually make things worse in the long run. It's an excellent nose clearer, and so useful for singers who are trying to avoid the old-folkie nasal tone that sounds as if you've got an Iron Jelloid up each nostril.

You can also use peppermint and lavender in a bath (careful with the peppermint or you won't stay in the bath very long, try 2-3 drops at first).