WHY DID I GET HIVES YESTERDAY? Need help please take the time out to reply!

Hello, I'm just going to cut straight to the point, yesterday I started getting really itchy and my skin was turning red. An hour later I started I get huge bumps all over my back and belly, if I itched a spot where their wouldn't be any lumps, they would come up, my throat felt like it was swollen to (I think that was just anxiety). Why did I get these? I read it's can be stress or an allergy, but it can't be an allergy as I didn't eat anything and my washing power was the same as it always is. I read you can get them from leukaemia and that scared me so much because obviously my anxiety has made me feel Ill anyway.

What caused my hives? I'm hoping it's just anxiety.