If it is a rash between the fingers, it could well be a variety of eczema as the folk above had said. First treatment is a moisturising cream, preferabley without lanolin that itself causes allergic rashes. After that fails, a weak steroid cream often does the trick and weak hydrocortisone cream is available over the counter (but as a diabetic person, you should get it on prescription for free). Beware moisturising creams with lanolin making rashes worse.
If it was between the toes, one would consider athlete's foot.
Various tablets cause rashes, but these are generally all over (can be in one area only) and generally a red like measles.
My partner hates the smell of steroid creams, so we use Boot's sting relief cream which is in fact very very mild hydrocortisone, and it works.
A small amount of a strong steroid cream is probably not going to raise one's blood glucose levels.
Best wishes