I have a painful hard knot in my right armpit and all down my arm into my hard, everything is throbbing like a toothache and constantly, what should i do?

The important thing is to identify it for appropriate management. If it started suddenly like within a day, it sounds like a boil/cellulitis, you may need antibiotics however you could relieve the pain by warm compress. The warm compress may also help it form a head quickly and drain but you should be examined for need for antibiotics since it looks like you also have cellulitis of the arm.

Seek medical examination, evaluation,appropriate treatment plan. We cannot diagnose you without examination, possible lab tests, possible imaging. Seek one on one care, not just a computer click.