What Causes Pain From Nipple Radiating To Armpit?

Hi, I am a 45 year old male, who recently started 1000mg of mitphormin for type 2 diabetes. About 3 weeks ago I felt a small pain travel towards my right nipple, like a spec of sand through a tube. I feel it has stopped under my right nipple. It hurts when I touch it. I know feel a little pain progressing towards my right armpit. Can someone give me some insight on what this can possibly be?