Thanks for asking at Healthcare Magic I read your question and I understand your concern.
Lump containing pus behind the neck has many differentials
1.
staphylococcus aureus infection
(short history, usually warm painful, tender)
2. cold abscess due to
Mycobacterium tuberculosis (long history, weight loss,
low grade fever)
3. abscess due to actinomyces Israeli
I would like to advise you not to be much worried.
Take simple antibiotic having stap aureus coverage like
Amoxicillin Clavulunic acid 1gm two times a day for 7-10 days. If lump still does not disappear, increase in size or you were losing weight more than 2.5 percent per week, more than five percent of your baseline body weight then go for the following investigations.
Do
complete blood count, ESR, chest xray, Pus histopathology and C/S.
Hope it helps best.
Don't forget to rate this answer and to close the discussion at Healthcare magic.
Take Your best care.
Regards
Dr Ejaz