What Causes Dark Patches Under The Armpits?

hello i have pcos, and suffer from dark armpits i also have dark dry patches under my armpits. i originally thought that maybe it was just dry skin but its not going away. i moisture it but it doesnt seem to work sometimes there really dark sometimes there not as bad can you help me as wearing some tops reveals them and im embarrased by the problem is it something to do with my pcos? also i have had them over a year so its not a new problem