Why are my Lymph Nodes Swollen?

I have swollen lymph nodes in my groin, lower back, and back of head(behind ear). I went to the doctor for a leukemia test (it runs in the family) and it came back negative for that and lymphoma, they said just a virus my body is kicking. That was two months ago and Occasionally the lymph nodes become noticeable again. I fear its a cancer we didn't screen for and it has reached my lymph nodes, does anyone have other ideas? *No other symptoms other than swollen lymph nodes and occasional stiff back*