I know my title is a bit odd, but its true. It is "normal" that some people have more sensitive skin and thus may form a rash, or a contact dermatitis as us medical people like to confusingly call it. I certainly can't speak for your situation exactly but many people have a small amount of redness, irritation, etc under the cast that can be slightly itchy, irritated, etc. Once the cast is removed I tell my patients they can rinse gently with soapy water but absolutely no scrubbing. If you developed a more serious rash that the residual adhesive (stickiness from the tape/cast) would need to be removed. Its likely you have more sensitive skin and I'd be surprised if it didn't fade away now that the cast is off.
I do think,t he most important thing to do is to contact your surgeon and have a look. Rarely there can be other issues that can cause problems so its always best if you are worried to ask. As a surgeon, we typically want to know when a problem is happening as we want to make sure its nothing serious and for you, we want to help put your mind at ease.
JPB