What should you do when a lump has appeared in your armpit, it hurts when touched but no color difference.?

Visit a doctor. A lump on your armpit could be a lot of things: like your body's reaction to an infection (your lymph nodes will swell up), a bug bite or something more. Risk factors for this being something serious are: if it is only on one side; if you don't have an infection; you don't remember any trauma there. Visit a doctor and be prepared to answer questions about family history of major illnesses.