Ehrlichia can also cause sinus problems...
So can viral infections...
And mold/fungal allergies are more common in winter, and dust allergies can occur year-round.
And some trees are starting to pollenate (yes, they start in winter).
I use Peaceful Mountain's Nasal Rescue - it is a colloidal silver nasal spray I found at a local health food store. It helps the unending sinus drainage/post nasal drip problems I've had for years (not from yeast in my case - I have very bad sinus allergies and bacterial infection problems).