Hello and thanks for the query
The lump is start paining, increases in size or color needs a medical attention and treated as soon as possible. Mostly under armpit the lump are due to enlarged lymph
nodes that has been caused by infection either bacterial or viral. Other causes may be malignancy, cyst
or tissue collection.
You need to get examined by your doctor and some investigations like FNAC may help you to get to a diagnosis. A course of antibiotic will help you if this is due to the infection. Else treatment depends on the cause.