I have had a swollen lymph node under my left armpit for the last three days, and muscle soreness, what could be wrong? -21 year old female

Viral illness: Lymph node enlargement differential diagnosis (ddx) is large, but likely an acute viral illness -- eg influenza. If not improved soon see a doctor. For ddx see table 3 especially - LAD a-z http://www.Matheny.Info/files/habermann_mayo_clin_proc_2000_75_723.Pdf.

Could be many things: Enlargement of lymph node can be caused by many things- like trauma/irritation around the area, infection - bacterial, viral etc, autoimmune disease, cancer etc. Judging from the duration of your problem- things like irritation/trauma to the left upperarm/arm pit- i.e. Shaving, boils, scratching over the skin on left arm etc- can cause the enlargement of the lymph node there.