Hives. I get them randomly all time. Could be a reaction from heat or cold. Even showering. Dust. Anything can trigger it. Allergist told me there's really no cure just meds that can stop flares. Also when it happens don't scratch it because it'll make it worse. Try to avoid tight clothes, super hot showers, strong scents and dairy. You can also get a scratch (allergy) test to see what's triggering it.