What Causes Itchy Rash On Legs, Neck, Chest And Armpits?

I have an itchy rash on my body. It sort of looks like razor burn, but it s also in areas that I do not shave, like my neck and chest. The only new items I am using are magnesium and iron supplements, skin care items containing extracts of peat, and a conditioner made with soy protein. The rash is most noticable on my legs and in my armpits, which is why I initially suspected that it was razor burn. But with it also on my neck and chest, I not sure what caused it. Any idea what caused this? And how do I treat it? Any help would be greatly appreciated, thank you.