Most likely, you have a mild viral infection (a cold) or allergies.
Normally most of our breathing is through our nose. The nose adds moisture to the air that goes into our lungs. With nasal congestion, you will have more mouth breathing that dries the surfaces of the mouth and throat. This can cause a prolonged sore throat. You bypass the normal adding of moisture to the air that the nose provides. This can make your throat dry and sore. Also post nasal drip from the nose or sinuses can irritate the back of the throat.
A sore throat caused by nasal congestion or post nasal drip is often worse in the morning when you first wake up. You might also notice a tickle at the back of the throat and coughing.
Do you have two or more of the following symptoms:
nasal congestion
runny nose
cough
a tickle at the back of the throat?