Up until very recently I had problems breathing through my nose especially at night when the left one would be totally blocked. I also had continuous post nasal drip. I discovered I had a sinus infection and was given Antibiotics and a nasal spray called Flixonase. They didn't stop my bad breath but they worked really well in loosening the mucos and keeping the airways clear. I continue to irrigate my nose and sinuses(I'm currently using tree tree oil and water until my Grapefruit-Seed-Extract arrives). the post nasal drip has receeded a lot and I have no problems breathing through my nose now.
Is it your nose that is sore or your sinuses - around the cheek bones and the corner and upper parts of the eyes? If it's the cheekbones and eyes it may be that you have a sinus infection. Either way if you are in pain you should see a doctor.