What could be causing hives?

i have been sleeping over my boyfriends house for 3 years every weekend, and i never broke out in hives before...he got a matress from his friend and nothing had happend to me until about 3 weeks later i started breaking out in hives...at first i thought he had bed bugs but we checked the whole bed and cleaned out his whole room and there was nothing, we even bought a spray for dust bed bugs and lice...every weekend he makes sure he has clean sheets and even puts a quilt over his bed for me, but i still break out in hives...i dont know if its the matress or could there be something in the room...why all of a suden its happening now, i cant go to the doctors because i dont have ensurence right now...does anybody have an idea what it can be from???