This has happened to me as well, with both of my kids on separate occasions.
They developed huge weals (large hives) all over. The doctor prescribed benadryl & lots of liquids. For both, the hives subsided in a couple of days & left as mysteriously as they came. We never found out why it happened.
The nice thing about benadryl is that it is a sedative, so the kids were able to avoid a lot of the itching & uncomforableness by being unconscious most of the time.
It made me recall that both my kids also developed newborn rash soon after birth. They developed hives then as well, although the newborn rash doesn't seem to make the infant uncomfortable.
Good luck with this, I hope this gets sorted out soon. How awful it is when we cannot do anything for our children.
Out of curiousity I peeked around a bit and found this...
• Cold. Sometimes cold temperatures can cause hives. The same goes for a sudden change in temperature — when your toddler's skin is quickly warmed up after being cold, for example.
here...