Oval red rash in armpit. Itches and im a diabetic. What could it be?

Many possibilities. There are many types and causes of skin rashes that could cause itching. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.
Many causes. Including allergic reactions, bacterial or yeast infections, insect bites.......Since you are diabetic it is important that you see a doctor and get it diagnosed and properly treated. Do not delay.
? Yeast infection. Keep the areas clean and dry (wear loose cotton clothes). Make sure your diabetes is well controlled- if your sugars are high, you'd be at risk for repeated infections (if your sugars are high, speak with your dr about how to improve your diabetes control). You might need prescription antifungal pill (depending if your dr thinks it is a yeast infection).
Yeast infection. It's very possible that. You have a fungal or yeast infection in your axilla. It's possible that you may have a contact dermatitis too. Avoid an alcohol based anti-perspirant. Keep the area as dry as possible and use an anti fungal (like lamisil). If it gets worse and there is more redness, pain and swollen glands under there, you must see a doctor.

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