Welcome to Health care magic forum.
You are a male,
obese, diabetic, used medicine for 7 years, and lost 100lbs, so there is a loss of good amount of the fat from the chest along with other parts of the body.
The collecting system of the breast though rudimentary, and the nerves, and
connective tissue of the breast will contract and drag the nipple.
While you are on thread meal, these tissues become stretched due to the vigorous movement, and cause pain.
Any how it is always better to consult a surgeon to exclude any pathology.
To be on safe side get the blood sugar checked, at least once in an year.
Wishing for a quick and complete recovery.