I am on a 'rash' rampage! My dgtr has a rash right now - doesn't sound the same 0 but no itching - just horrifying hive like things that the MD termed Urticariel Multiforme (?? I have no info on it!) Apparently benign - but it's just crazy looking...
But - my friend (I have 2-3 w/ children ages 2-4 w/ rashes right now - none of the kids see each other!). Her 4yo has a rash like you describe I think and it is Pytoriasis Rosea. Look that up - I guess it is just a rash that has no tx, is viral in nature and could take weeks to go away.
As for the diarrhea - that is a little disconcerting. Has she had any antibiotics recently? She really should be seen by the pediatrician tomorrow (Friday) for that at least. It can quickly be dehydrating and you don't want to work on rehydrating.
Hope she feels better.