I get itchy all over any time I get warm too. Like you, examples include a hot room, exercise (especially in the sun), laughing too hard, etc.
If I get especially warm, I'll break out in hives on my stomach and chest. This used to happen every single time I took a warm shower, but now I generally just get itchy and don't develop the hives.
This started when I was about 16 and has continued to varying degrees for 20 years now. My Crohn's issues started around the same time, but it's not clear whether they're related or it's just a coincidence.
My condition is thought to be "cholinergic urticaria," if you want to google that.
I take zyrtec every day and it helps some. I still get a bit itchy but it's not as bad.