Continuous Cough, not getting better.

My mother is having sever cough problem since last 3-4 weeks , though we have shown her to multiple doctors and even tried homeopathy , but she hasn't got any relief and none of doctor able to tell the act reason for her continuous cough, as most of them told that it's due to allergy and given medicine for the same , but still she didn't get better. She has a conjunction in her chest and get a attack sort of coughing which continues for 4-5 mins , especially in night and she feels very tried all the time . She is a diabetic since last almost 9-10 years and is on insulin twice a day from last 1 1/2 years . Please advise the exact reason for her cough and which doctor do we need to consult like GP,ENT , internal Medicine or any other and even please advise if any test also need to some to ascertain the cause for cough.