Glands under the shoulder arm in a 9 month old baby can have variety of reasons.It can be a perfectly normal for a small increase in the size of the lymph
nodes or it can be a cause of concern.
More common causes are infection of skin or other body parts, tuberculosis
, or some tumors can also present with this.
To reach a conclusion it needs to be checked by a doctor for its size, consistency hard or soft,presence on the other side or other body parts, increase or decrease with the passage of time, any other features like fever
associated with it.
Please consult a doctor and get it checked.