I a lymph node in my groin is swollen but got smaller after 5 days of antibiotics 2 weeks ago. No other swellings. Do cancerous nodes get smaller?

NO. Cancer nodes shrink with successful treatments not on their own , most common cause of groin lymph node enlargement is infection in lower extremity , genital and anal infections, some time very mild , not noticeable infections, just don't worry follow your doctors instructions.