o also i should have said cancer is not usually painful to start with, and symptoms include lump, discharge/blood, change in breast size/shape/colour, and im guessing coz u dont have any of the symptoms thats why they said its nothing.
how long has it been hurting under your arm? is it central armpit? because breast pain isnt normally in ur armpit because the lymph nodes are usually only on the edge of the armpit and if it was a breast related problem that was giving u armpit pain then ur lymph nodes would probably be swollen. has ur actual breast been hurting? if it has regularly, then go back to the docs and say your still worried. but at ur age breasts may get more tender at certain times of the month etc, usually before periods. it may be muscular, certain sports can cause muscular pain. go and see another doc if ur worried for a second opinion, but i guess if u dont have a lump or any usual breast symptoms they wouldnt refer you.