Itching

About two weeks ago, my right arm started itching.  I have had this flare up in the past and it usually affects one arm or the other but never at the same time and the itching seems to be in one area of my arm anywhere from my upper arm to my wrist.  I did go to the dr. and he prescribed Zyrtec and told me to use Benedryl cream.  I have been taking the Zyrtec and using the cream, but it is not helping at all.  My arm is itching pretty much all day, but it seems to get worse at night when I go to bed which is now causing me to lose sleep.  I have been feeling stressed for the last six to eight months and I'm pretty sure the itching is due to that, but why did it just come on out of the blue and what can I do to make it go away.