I have drainage down the back of my throat, is it allergies or sinus?

Either...or both. Not enough information to make a diagnosis but i suspect some friends and relatives are trying. Here's a clue: allergic noses itch & sneeze while coughing is a sign of sinusitis. Watery runny nose? Think allergy. Thick goopy stuff? Sinusitis. Allergic rhinitis predisposes to sinusitis. Time to see your doctor?
Post nasal drip. Post nasal drip is a very common occurrence in both allergies and from infections. Thus, the symptom of post nasal drip does not point towards one over the other.