Recurring hives w/o clear reason?

I have been getting hives daily since X-mas. That day, they were rather severe on my back and down to my thighs and dinner plate size. The last two nights (Including tonight) they have reappeared on my hips, upper arms and sporadically on my back. Not nearly as large, but quite consistent and they always go away overnight and return in the afternoon. They disappear entirely in between. Maybe one little one will stick thru the day. Nothing has changed in my environment, and I am not overly stressed. Anyone else had mystery hives? What did you do? I have never had them prior to this, so I a trying to figure out what to do w/o visiting a doc.