Often, armpit pain is a simple case of a strained muscle. "The most common causes include repetitive or traumatic injuries to the muscles and tendons that insert onto the arm bone," says Lin.

These can be strained due to repetitive movements or overhead sports, such as swimming, volleyball and baseball, says Lin. Armpit pain can also be caused by acute injuries to muscles or ligaments ...

Often, armpit pain is a simple case of a strained muscle. "The most common causes include repetitive or traumatic injuries to the muscles and tendons that insert onto the arm bone," says Lin.