I was going to make the same suggestion as Other Mummy ~ try Benedryl and if it 'calms' than you know it is likely some sort of allergic reaction to something and you might never know 'what' .... could be a bug bite, a weed or plant you touched outside, combination of foods you ate, pesticide on a food or something air born that you just 'breathed' in out and about in the community

If Benedryl does not help it definitely go to the Dr and get it checked out ... sometimes we get 'blood infections' from a tiny cut you would not even think was an issue and it can present in a 'rash' ... have you been checking yourself for 'fever' as well?