Severe Allergy- need to identify a trigger

I have woken up this week (on tuesday) with hives covering my body. I new that this indicated an allergy, and as I had been away at the weekend, I tried to not worry and just took an antihistamine (as I thought it would be something I had come into contact with at the weekend perhaps)
I then woke up the next day, on wednesday morning, with my body covered head to toe in hives, my joints started to swell, along with my scalp, my ears, lips, eyes, hands and feet. I went to the doctor, he confirmed it was an allergy and proscribed an antihistamine.
I have had to take time off work already and it is just something I cannot afford to do. I am so worried, as I've woken up today (saturday) after I thought I'd been through the worst of it, and I've got a pounding head ache, and the hives and swelling are starting to reappear.
I do not know what is causing this, and because I don't know what  it is, I do not know how to avoid it. I am so so worried...please help.