Lymph nodes under jaw are swollen

The lymph nodes under my jaw are swollen, One more than the other. I feel fine, had a cold but the nodes were swollen before hand as well. They are hard, rubbery feeling, and are easily movable, You can't see the them but I can feel them. They feel like they are about the size of the fingernail on my thumb. I am really scared. I am 20 years old, I have not had any serious illness before, my grandmother has breast cancer, but that's it not other history of it.

I read that if the nodes are hard that that means something is really wrong. Is that true? Why are they swollen?