@SarahGen-- I wonder if you have a food allergy or intolerance?
My sister had this problem and it turned out that she's allergic to dairy. Since she continued to have dairy products, the post-nasal drip wasn't going away. She quit dairy and her symptoms disappeared. Even if you're not allergic to dairy, doctors don't recommend it when there is post-nasal drip and sore throat. So try avoiding it for a few days and see if that helps.
Also, try gargling with salt water or vinegar water. These break up congestion very well. You can drink hot tea or use cough drops if you have a sore throat. Get well soon.