What Causes Itchy Patches On The Lower Legs After Consuming High Sugar Foods?

I used to get an excruciatingly itchy patch on my lower legs which was brought on usually by eating sugary foods. Prednisone tablets and cortisone cream fixed it. 2 weeks ago I got a big red circle with a small black dot in the middle. I thought it was the same but it got worse. Then thought maybe white tail spider. No. Went to the doctor as thought it might be cellulitis. Penicillin isn t working. Half way through packet so much be a dermatitis. What do you think? Currently have a cold compress on it as it is extremely itchy. No discharge. Just big red lump with small dot in centre. A bit peeling . I want it fixed.