What are these itchy patches of skin?

Just yesterday I developed a horrible rash. My patches look like they are swollen, like welts. Its like all over my body and changes places. I didn't eat anything I've never had before and I didn't even leave the house. No pets in the house either. What could have caused this? What is it? And how can I get rid of it? I bought some Allergy tablets, antihistamines. Thanks