Just 3 days you probably have only a reactive lymph node ...
Even though you feel sick it is probably just associated with activation of your immune system to some infection ...
You may get your doctor to check this for a baseline ..
3 days duration so far to me does not suggest anything ominous as I think the point of your question implies ...
Have your doctor examine this node and check neck exam/ throat / sinuses / lungs etc....
A firm node should be generally 1.0 cm or less in size and a reactive node usually recedes gradually ( 6 weeks or so common)
It is not such time to want you to get overly worried or subject to unnecessary procedures _( such as excisional biopsy for example)
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