I'm an expert on the ivy. I got it so bad I almost went crazy. Seriously, 5 days with NO SLEEP. Anyhow.
If you've got it really bad, you'll need to go to the doctor and get a oral corticosteroid prescription (pill). I've heard of the shot as well, but my doctor hadn't, so he didn't give it to me. Note: If they don't give you the correct dosage (ie: too smal), the ivy reaction will seem to go away, only to rebound more severely within a week or two. So make sure they take it seriously.
As for the spreading, you may have just gotten it ALL over you initially, but the derma (skin) is thicker in certain areas and it takes longer for those areas to react, so the "infection" seems to spread. The urshiol (the active oil ingredient in the Ivy) bonds to the skin within 2-3 mins. But if you had it all over your body, and then you took a warm shower within that time frame....it most likely spread around even further.
Another thing to be aware of is the oil from the ivy remains on ANYTHING you touched after you first came in from the outdoors. If you pet your animals, it could be on their fur (animals don't react, but the oil stays potent for as long as it's there) This could be another source of continued reaction.
You'll need to wash anything you make have touched after coming into contact with the ivy. That means door handles, etc.
mrC