The cough is probably caused by phlem running down the back of your throat; and it is probably an amount that you don't notice.
I had these symptoms a couple of years ago. I was worried because I had to do a lot of flying soon. If you've ever flown with a sinus cold, you know what I'm talking about. My doctor said he could give me meds, but if it was a virus, antibiotics wouldn't do a darn thing, so he suggested I try something before he did that. This is going to sound weird, and it did to me too, but he suggested I get a nasal wash system. Not a simple nasal spray, but a wash that irrigates your sinuses. I've never had a cold/sinus infection clear up faster; even with meds.
Some people use what is called a netipot, but I prefer one with a bottle that forces water through a little more. I know it sounds uncomfortable, but you will be amazed at how good it feels and how well it works as long as you follow the instructions. It is fairly inexpensive and I highly recommend it for when you have symptoms like that. Here is the one I use; it was like $13-15 at Walgreens:
http://www.neilmed.com/usa/sinusrinse.php
I'm telling you, it will clear you up in a few days or less if you use warm water and the solution packs included.