Today for the first time i felt a painful bump under my armpit. it is about the size of a pea bit is very tender?

Pustule: This is most likely an infected hair follicle. Apply warm compresses. If no improvement or if increase in swelling,redness,pain or drainage of pus, consult a physician for evaluation and treatment. In the meantime cover with gauge affixed with paper tape. Prevent contamination of surrounding area. You could also apply an antibacterial ointment like bacitracin.

Infection likely: A small tender area like you describe could be an abscess (treated with frequent warm compresses & possibly antibiotics & later drainage if needed) or a lymph node reacting to an infection in that area (would need a Dr exam to evaluate). Using warm compressed 10 mins every few hours would be reasonable to try for 1-2 days if no fever or severe pain, but call Dr if worse or no better