A scratchy or sore throat can be caused by an allergy-induced postnatal drip. Once exposed to an allergen, your sinuses may begin to drain, resulting in a scratchy discomfort.
Your body is simply increasing its mucus production, which is why you may also experience a runny nose. Although the mucus itself does not generally cause pain, a sore throat is often experienced because individuals try to clear the mucus, resulting in burning and discomfort. To soothe a sore throat, gargle warm salt water and drink plenty of fluids.