Laura, what you have described is a palpable occipital lymph node. These are normal and can become swollen for a whole variety of reasons. I advise you to educate yourself about your lymph nodes and if you feel this node is growing or more nodes pop up then see your physician for treatment. Typical causes of nodes in that area are : Flu, infection in mouth, trauma, lymphoma(cancer),or other type of inflammation in upper body.
...more info http://en.wikipedia.org/wiki/Lymph_node