Wow. I had cold uticaria when I was a teenager. I've never met anyone else who had it (in fact, I thought I was crazy until a doctor told me it was an actual allergy). For me, it didn't just occur in winter, though. Anything that was remotely cold or cool would trigger a reaction. Even going from a hot to a cooler (but not necessarily cold) environment would trigger it, walking outdoors in the evening during summer with bare skin exposed, any wind at all that cooled my skin, touching relatively cold items outside such as metal or sitting on cool cement, holding an item just removed from the refrigerator ... Even taking two Benadryl beforehand I was unable to swim in cold ocean water (I'd get out after 30 seconds covered head-to-toe with hives). The one time I tried swimming in an outdoor pool without taking an antihistamine beforehand it triggered a full-blown anaphylactic reaction and I lasted ten minutes (ignoring the hives) before I collapsed from my blood pressure plummeting. That turned me off swimming outdoors for YEARS, even long after the reactions had (thankfully!) disappeared.
I've had problems with allergies my whole life and I would consider it a sort of autoimmune issue, except that your body is reacting to a harmless substance that doesn't happen to be part of your body. I've had a severe food allergy my entire life, but it started out when I was young as hives showing up hours after I'd eaten the food. It wasn't until reactions started happening immediately (and became anaphylactic in nature) that we were able to connect the food to the reaction—and it wasn't one of the common allergies like nuts or gluten. I developed a ton of seasonal/environmental allergies suddenly when I was 24, and I've noticed that when they are bothering me a lot, it's like my body gets ultra-sensitive and reacts to any irritant at all, and things like fabrics or friction can cause hives (and I also have minor reactions to foods that don't usually bother me).
I think one of the hard parts about allergies is they can be so hard to pin down and they can also change really suddenly. I hope you are able to find some answers through Children's Hospital—all experiences I had with them as a kid were excellent, so I hope the doctors there are able to help!