What are some causes of swollen lymph nodes?

Many.... The most common cause of swollen lymph nodes is infection (bacterial or viral). It sometimes can take 1-2 months for the enlarged lymph nodes to return to their normal size after an infection. If the lymph nodes continue to grow, occur in different areas of the body, or you start getting sick symptoms (fevers, unexplained weight loss, night sweats, fatigue), i would go see your doctor.
Many things. A bug bite, an infection, a virus mono and, many benign conditions can cause a swollen lymph node. Keep an en eye on it and if it is still there or getting bigger after a few weeks, get it checked.
A lot of things. Infections are the most common cause, either from bacteria or viruses. Cancers and a large number of other disease can also cause them to enlarge. Most of the time lymph node enlargement is not due to cancer. Your doctor can check them out and help.