My little sister has tiny rashes all over her body?

My little sister(10 years old) has tiny rashes all over her body ranging from her legs armpit chest and her groin area... She went swimming for 2 hours yesterday... And she has been complaining about itching.. Could this be the start of chicken pox? I'm the only one in our side of the family that hasn't been infected before AKA I have no immunity... But I've had the vaccine but that's been proven to fail when my older brother got in when he was a baby... She hasn't been in contact with any poison oak or ivy... We were raised in Kentucky and have been affected by poison ivy soo many times I'm sure it's not that... What could this be?