What Causes Swollen Lymph Nodes On The Side Of The Neck?

Just recently realized that my neck (on right side) had a swelling that was just determined to be a cluster of enlarged lymph nodes (2cm). I had an ultrasound today and plan to do a procedure to check it out. I know that I have not been to the doctor to finalize cause but I am very concern.

I have no history (personally or family) of what can bring this on. I stay in shape and eat healthy (know personal habits that would compromise my health). This has been in my neck going on three weeks.