Protein rashes (hives/urticaria) are common among people who go on a high protein diet. They are allergic reactions on the skin to a changed macronutrient profile (protein, fat, carbs ratio) in your new dieting routine
During an allergic reaction, the body releases a protein called histamine which causes the surrounding blood vessels to dilate and increase permeability. This allows fluids and immune system cells (white blood cells – leukocytes) and blood plasma proteins to leak from the bloodstream through the blood vessel walls (because of increased permeability) to the nearby tissues to fight inflammation. This fluid accumulation is what causes the red rashes and irritation.
Hives can also happen when a lactose intolerant person includes whey protein in the diet. Switching to whey isolate, instead of whey concentrate can help in such a scenario. Not all protein-rich food may cause protein rashes. It could be due to a variety of issues, sometimes including less water/fiber intake, indigestion, etc.
It is important not to make sudden & huge changes in your eating habits or increase protein dramatically. Any change in the lifestyle has to be introduced gradually and steadily!