The rash you most likely experiencing is mercury. Sometimes something you've done can release a lot of mercury into the areas where it is able to be excreted and it can come out onto the skin. When a person baths/showers, or exercises, it will usually increase further because heat makes mercury rise. And it can take sometime to die down.

Taking things like vitamin C, selenium etc, can make this even more intense because if the mercury is in the areas where it's coming out onto the skin, this usually means it's readily available and won't take much to come out and cause a lot of rashes, sometimes covering the entire body, usually mostly around neck and chest, but has been known to come out even on ears, scalp, hands, the entire thing!

It is a good sign, though very annoying. It takes a load off the liver and other organs when it comes out this way.