Just being on a bland diet doesn't mean you don't have food allergies. I'm sensitive (not "truly allergic") to lots of things that are bland.
Start keeping the often-referenced food diary (put keywords in the search box at the top of the page), which includes food, drinks, medicines, pollen in the area and anything else you can think of. It'll be pretty hard to figure out time of reactions since you have the hives all the time.
I'm sensitive to colors in medicines and liquid meds, particularly Benadryl. I'm fine with the white tablets. I haven't used my Epi-pens yet, but I know I'd react because of the preservative. My particular sensitivity is to sulfites, and I'm lucky because lots of people have the true allergy to it, with hives and the works. But that's my allergy, not necessarily yours.
I had a bad reaction to an IM shot of hydroxyzine, which may have been due to the preservative or the gelatin carrier in many shots and IV meds. I have no problem with little white meclizine tablets (related). I also react to immunizations because of the preservative and gelatin.
I found my allergen/food trigger with the help of my allergist who had me keep the food diary. My bland foods include gelatin (also in many meds), corn starch (filler in meds), sugar, particularly brown sugar, maple syrup, commercial breads including from bakeries, most cereals, especially if they contain molasses, corn syrup ... So you see, bland doesn't mean no allergy.
There's a good story from Housemouse about
her skin allergy - like yours. She nearly died until she found her trigger. She is in the links section of www.learningtarget.com/nosulfites/index.htm
or see www.housemouse.com/sulfites.htm
and scroll down to her story before reading her cures.
Housemouse's trigger is also sulfite, but the story of how to find your trigger works no matter what your's is.
Just an idea. Not a doctor.