There are a number of causes according to WebMD -
"What causes swollen lymph nodes?
Lymph nodes often swell in one location when a problem such as an injury, infection, or tumor develops in or near the lymph node. Which lymph nodes are swollen can help identify the problem.
The glands on either side of the neck, under the jaw, or behind the ears commonly swell when you have a cold or sore throat. Glands can also swell following an injury, such as a cut or bite, near the gland or when a tumor or infection occurs in the mouth, head, or neck.
Glands in the armpit (axillary lymph nodes) may swell from an injury or infection to the arm or hand. A rare cause of axillary swelling may be breast cancer or lymphoma.
The lymph nodes in the groin (femoral or inguinal lymph nodes) may swell from an injury or infection in the foot, leg, groin, or genitals. In rare cases, testicular cancer, lymphoma, or melanoma may cause a lump in this area.
Glands above the collarbone (supraclavicular lymph nodes) may swell from an infection or tumor in the areas of the lungs, breasts, neck, or abdomen.
Common sites for swollen lymph nodes camera include the neck, groin, and underarms.
What does it mean when lymph nodes swell in two or more areas of the body?
When lymph nodes swell in two or more areas of the body, it is called generalized lymphadenopathy. This may be caused by:
A viral illness, such as measles, rubella, chickenpox (varicella), or mumps.
Mononucleosis (Epstein-Barr virus), which results in fever, sore throat, and fatigue, or cytomegalovirus (CMV), a viral infection that causes symptoms similar to those of mononucleosis.
A bacterial illness, such as strep throat (caused by the streptococcus bacterium) or Lyme disease (a bacterial infection spread by certain types of ticks).
Side effects of phenytoin (Dilantin), a medicine used to prevent seizures.
Side effects of measles-mumps-rubella (MMR) vaccination.
Cancer, such as leukemia, Hodgkin's disease, and non-Hodgkin's lymphoma.
Acquired immunodeficiency syndrome (AIDS), which develops after contracting HIV (human immunodeficiency virus). This virus attacks the immune system, making it difficult for the body to fight off infection and some disease.
Syphilis, a sexually transmitted disease."
Any swollen lymph nodes that don't go away or return to normal size over about a month should be checked by your doctor.