Brief Answer:
Consult ENT specialist earliest.

Detailed Answer:
Hi XXXX,
Thanks for the query under health care magic. I understand your concern.
It seems you have fulminating infection of sinus leading to swollen lymph nodes in neck ( causing stiff neck ).
You had taken three courses of antibiotics , don't seem to control the infection. Probably bacteria affecting are resistant to the antibiotic used./ infection is not being control due to conditions like low resistance or uncontrolled diabetes.
I would in this context advise you to consult an ENT specialist.. who would examine you, get you investigated ( CBC,Culture & sensitivity of the phlegm, Blood sugars, allergy screen test ) to find the cause of non response & also suitable antibiotic to control the infection . He may add decongestants ( oral, nasal drops ), anti allergic pills, anti inflammatory drugs to the treatment as per need.
A full course of medication suggested by expert along with few life style changes like having high protein & vitamin nourishing diet, plenty of warm water, avoiding allergen( if any)/ avoiding cold, sour, fried eatables/ avoiding exposure to cold weather or dust, fumes. ... would help you to get rid of the problem.
I hope the answer was informative & helpful to you. Any follow up query is welcome.
Thanks.

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