I am so sorry, but I have no clue as to what could be causing you these issues. However, if the Prednisone worked, perhaps you can talk to your doctor about staying on a lower maintenance dose. Have you seen an endocrinologist about this issue in your throat? It could be your thyroid. Has any doctor checked your TSH levels to see if your thyroid is behaving normally.
I would not give up on this. You may have to insist that further tests be done in order to find out exactly what this issue is. Just because the doctors are not alarmed or they feel it is nothing to worry about, doesn't mean that it is nothing to worry about. Get aggressive with your doctors and insist that they do all that is necessary to find out what this is and how to treat it.
I wish you the very best.
Peace and Blessings
Look For The Good and Praise It!