What Could Cause Itchy, Oval-shaped, White Welts On My Arms, Palm, And Waist That Turn Red When Scratched And Improve With Benadryl?

I have whelps, not hives that come up on my arms, on the palm of my hand, and around my waist. They are not red until I rub or scratch them. They often look very similar to a large mosquito bite or even a horse fly bite. They are usually oval shaped and more white when they first appear. They itch like crazy and usually go away if I avoid scratching. Benedril does help.