What could be the cause of my swollen lymph nodes?

Many possibilities. : Possible causes are: 1) infection- viruses, e.g., mono, herpes, hiv; bacteria e.g., strep, staph, etc; inflammation from autoimmune disorders 2) primary malignancies e.g., lymphoma, hodgkins 3) secondary malignancies e.g., metastatic cancers- breast, lung cancer, others 4) rare inherited metabolism disorders 5) rare poorly characterized disorders e.g., angioimmunoblastic lymphadenopathy.