Dog With Runny Nose

I have a soon to be 16 year old Silky Terrier and he has over the last year developed some kind of nasal congestion.  I've taken him to the vet and he's had all kinds of pills and shots but I know long term use can be harmful.  The strange thing is that he only gets this nasal congestion in my bedroom.  I've pulled the carpeting, have started using hypoallergenic laundry detergent and I don't keep any cosmetics/perfumes there.  Any idea what could be going on?  I thought it could possibly be dust mites.  Anyone out there experiencing a problem like this?
 
Thanks for reading!