S.,
My 4-year old was diagnosed with urticaria pigmentosa at 2 1/2. It was right around the time that my youngest was born, and my m-i-l had given her a bubble bath, to keep her entertained, while I was still in the hospital. She loved it so much, we started giving her bubble baths all the time (I'm generally a shower kinda girl), and that's around the time that her spots started showing up/multiplying.
Not long before all that, she woke up one morning with what we thought was a spider bite on the inside of her elbow (we thought maybe she had squished a spider, bending her arm in her sleep). The dermatologist thought it was just one of her "spots," kind of oval in shape, like the rest, just irritated more by it's location. I think I later read somewhere that U.P. can be brought on by venom, so in my mind, that's still a possibility. I also wonder if there was something in the bubble bath soap that was irritating her skin, as well. (Keep in mind - I'm no professional.)
The dermatologist explained that there was nothing that she could really do to help, other than giving us some ointment, to help keep the "spots" from (I think) getting crusty. We also ran a CBC to make sure her counts were okay (since this disorder can also affect internal organs). She also advised us to shower instead of taking baths, and don't use hot water, as that will irritate it. She also advised that we be very gentle when drying her off, because brisk rubbing would irritate or increase the spots, too.
(Though they're mostly cleared up, you can see a few of her spots at http://www.frensley.org/gallery/2138659_UbV5V/1/110815501...)
After probably 4 months of showers only with very mild soap and gentle drying, there was no trace, other than a small scar on the inside of her elbow, where the "spider bite" had been.
I was frequently asked if she had chicken pox or if she was contagious. I'd explain that "she's not contagious, but has a skin disorder that makes her skin cells go whacko and overproduce, causing the spots." I'd usually get an "oh, okay," and we'd go on with life.
I hope yours goes as easy as ours did, and clears up as quick if not quicker.
Congratulations on your newest addition!
M.