Would this lighten of the legs and arms be caused by urticaria?

I don't think so: If what you mean by "lighten of the legs and arms" is lighter color of the skin, then no urticaria does not do that.Urticaria is a skin rash which can be caused by many allergic and non-allergic causes. You should see a dermatologist rather than making your own diagnosis.

Not sure: I assume you mean '"lightening" of (the color of) the legs & arms.' Most of the times urticaria causes a reddish discoloration or a darkening of the skin tone (especially in dark-skinned people). However, some contact urticaria can cause lightening of the surrounding skin. Without a picture or a personal exam, it is hard to say what is going on. Skin can lighten b/c of vitiligo & chemicals as well