I am 54 female. I have a red lump on the underside of my breast that turned dark purple/blue color.  It is hard yet rubbery feeling and now the skin has come off.  I was diagnosed with pre diabetes last year and I am supposed to take metformin 2 times a day but I am not good at remembering to take it.  I had a mammogram and the radiologist thinks it is cellulitis and my gyn has put me on a second round of antibiotics.  I am worred about cancer or an infection going to my organs.  I need reassurance.
Since you are a diabetic you need to maintain strict medication and diet restriction to maintain sugars under control. Infections are very common in uncontrolled diabetes and need prompt treatment. You said the skin over lump is red and is peeling of. All these point towards infection has strong possibility, and it would subside with appropriate antibiotics. Please remember all breast lumps are not cancerous