Hi, I Have Just Returned From Mexico, Punta Mita. A

Hi, I have just returned from Mexico, Punta Mita. A small rash started on my lower legs on the last day. When I returned home the rash hash spred all the way to the top of my leg. It is mildly itchy. I am not noticing the rash has spred to my arm. Do you know what might have caused this and what treatment would help?