Hello, A rash reaction on your face would be a possibility. Is your rash around your mouth or limited to the side where your filling was done? Do you notice any reaction inside your mouth? This could indicate a sensitivity to a material used at the dental office. Does your dentist office wear latex gloves? A rash on the face is a common reaction for latex sensitivity. Most dental offices limit the use of latex. Nitrile gloves or vinyl can be worn. Latex sensitivity should be noted on your medical history if you find this to be the cause. An allergist could confirm the sensitivity. A medical history with controlled diabetes raises some concerns with dental treatment. Surgical procedures may be at risk for delayed healing, but restorations and fillings, commonly, are not of major concern with diabetes. Uncontrolled diabetes can show stress on the tissues of your mouth and compromised overall oral health that would require greater attention. You may tend to have a greater amount of inflammation with your gums and tend to not resond as quickly to gum treatment therapies. I recommend discussing and describing your condition with your dentist to decide if you require further evaluation. Some trauma to the soft tissue of the face may be experienced during dental treatment and cause redness that should disappear fairly quickly. A true rash, Indicating an allergic reaction, should be investigated further. Additional exposures may elicit responses with increased severity and risk of anaphylaxis. Should your symptoms increase, consider a visit to your physician. This may not be related to your dental care and just a coincidence. I hope my suggestions are useful to assist in your diagnosis. Thank you for your inquiry. I would be glad to answer additional questions if you can provide me additional details concerning this rash.