How To Treat Rashes On Arms?

O.k. I have a question I already have a son and when I was
pregnant for him I had throughout my pregnancy a rash on
my arms. So before my cyle came today this month I noticed
that same rash developing on my arms again, today it's severe
and my period is not to heavy I took two pregnancy tests
one today and one yesterday both negative. Should I now
wait another week to take a pregnancy test or see how this
cyle evolves?