What Causes Itchy Rashes On The Chest And Underarms?

I started with a itchy rash at my bra line and tricep area that is also on my sides of chest and underarm. It is raised but kind of flat and reddens when I scratched it. It is a cluster of smallish 2mm-3mm and one is very large 5mm. I had chicken pox as a child but have been recently exposed to a case in my school. It does not look like there is a watery fluid inside the papules. One of my students had a spotty rash like this and went to her pediatrician who said it was a virus working its way out. I have been achey and tired and having some cold symptoms but no fevers. PS I am a school nurse at an elementary school around children alot so what should I do?