You obviously need someone to look at your back. We cannot tell you without that. Of course, other warning signs would be warmth (is it warm to touch?), tender, raised, producing pus, fever, and sweats. Since you are a type 1 diabetic, you might also notice your sugars being harder to control. Also, because of your diabetes, if this is an infection, you need to have it checked out right away since an uncontrolled infection can mess with your sugars, but also be more aggressive in someone with diabetes. Good luck, and go to your doctor.