It's not itchy, more a bit tingly/burning feeling down my arm on and off but when I googled images this is quite close to what it looks like but more spots (even though I think it's a baby/child's arm in the photo). It's not a large spread out area, on the top over to the body side of my arm.
http://liz.mommyslittlecorner.com/2010/05/prickly-heat.html
Randomly though, if anything my other arm has been hotter as I've had it tubigripped up as I bashed it when I fell the other week.
It's markedly raised (the little bobbly spots, not the whole area) and quite irritable if I touch it.