How common is armpit pain in early pregnancy?

Usually there is-: -breast enlargement, and there is what's called the axillary tail that can reach to just under the deltoid muscle, and this may be what is causing your pit pain, because in using your shoulder the deltoids come in to play & it could compress the axillary tail of your breasts.. no good image so Google the anatomy of the axillary tail of the breast or just breast anatomy.