What are the doctors looking for?

I went in to see my doctor because my neck was swelling around my enlarged lymph node and I was having a slight fever and headaches. After feeling my neck and asking if the swelliblng was painful ( mostly numb but throbs occasionally) he asked if I was lethargic and tired all the time. I said I was. He ordered an ultrasound on my neck and set the appointment up for two days later. What is he looking for. I'm only twenty. Thank u.