Why am I getting hives everyday but doctors say I'm not allergic to anything?

I have been getting hives everyday for almost a year. I've been to a dermatologist and an allergist. they did a blood test but it came back normal an sent me home with a bunch of samplers of zrtex. well that stopes working. the dermatologist gave me a prescription of hydroxizine witch is an antihistamine an a sleeping pill (it's hard for me to sleep at night cause I'm constantly scratching myself) it would really help an clear it up the next morning. I'm so sick an tired of getting an having these hives everyday. I would love some help!!