Two unhappy days with hives

On Saturday afternoon something happened at work, still not sure what, that started me going on a semi-severe allergic reaction. I had bright red and pink hives spreading out from my neck, down my shoulders, up the side of my face etc... Benadyrl from an ER doctor put it down for about six hours then it continued it's march across my face, arms and stomach. Everywhere the rash spread cleared up four to eight hours, leaving only a slightly reddened surface behind. The rash continued in an 'orderly' downward and outward spread from the original point. That was actually the most confusing part, that it was so regular in pattern. I went to my doctor on Monday morning. She gave me a steroid shot. The hives have mostly subsided now but it was definitely something I don't want to happen again. Doctor couldn't explain the cause of it. She said most cases like this if the person doesn't know themselves what triggered it a doctor can't pinpoint the cause without allergic reaction tests. The pattern of spread was 'systemic', something following the bloodstream?