What Causes Pain In Armpits?

Hi Doc, I have this pain under my arm pit. Its stinging. It was first in my right arm pit, then now is in my left arm pit. It used to come before my monthly period, but now it is there a week after and in both arms. Am worried... who should I go to?
I am a 25 year old female, considerably healthy (no chronic illness, I rarely get ill) I am50kg, lean, and about 5 feet (not so sure).