What are the causes of swollen lymph nodes?

Many things: Lymph nodes are one of the ways in which our body tries to eliminate harmful things, such as infection or cancer or other forms of inflammation. When they are active in this process they may become larger and be felt where they were not felt previously.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.