What is a hard lymph node swollen on my right collar bone?

Reactive: The vast majority of swollen lymph nodes "react" to viral infections. This is their job description. They enlarge to help fight the infection. It it's larger than a marble or has been their for more than 2 weeks, have your doctor take a look at it.

Need to examine: You may feel the node every couple of weeks, if the lymph gland doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor.