Many of my clients have had the same problem which seems to appear most often after they have eaten things with milk, wheat, coffee and or something with Sugar in it.
Many people are allergic to undigested milk proteins and wheat proteins such as gluten and casein.
Many of my client don't which one is causing the allergic reaction but they have complained of severe itching on the arms, shoulder, neck and sometimes on the legs particularily on the inner thighs. It doesn't seem last continually but flairs up everytime they eat the above.
The only suggestion I can give you to hopefully solve this problem is to go on a three day weekend water fast. Only drink distilled water with some squeezed lemon in it. You must drink four onces of water every half hour during your waking hours.
Once you have fasted for three days introduce three new foods, one at a time ie: only a glass of milk or only one piece of toast. Allow at least three hours between each food group. Write down which food group brings on a reaction. Remember, only one food at a time. Do not mix.