The more common reasons for armpit pain are swelling in the lymphatic glands in the armpit and breast area, or a pull of the Pectoralis muscle which then irritates the ribs underneath.
Shaving too close to the root, or beauty products may also cause a severe allergic reaction.
If the pain does not disappear, it could be symptomatic of something more serious, and you should go to your GP and have it palpated and examined.
