I cannot think properly when my nose is blocked.i have deviated septum along with nasal allergy.i face very severe difficulty in thinking/concentrating when exposed to a/c.is this difficulty is due to deviated septum and nasal allergy?

You perform at your best when you have adequate restorative sleep, are well nourished, are healthy and in a comfortable setting. Illness, whether as allergy or other factors reduces effective productivity. Get this managed and things should improve to your baseline abilities.

Obviously to be diagnosed you have to be examined. Symptoms alone are not diagnostic, but must be folded into a physical exam, health history, medication history, perhaps imaging, perhaps lab tests. Please seee your General Physician for a complete evaluation.