It sounds like a manifestation of Chronic Spontaneous Urticaria. You said you used to have the occasional random outbreak of hives before.
That is what happens with CSU. You can have an episode, that lasts as short as a few days, then it goes into 'remission' then when your body is under some stressful event (a cold, an stressful emotional event) it reappears. It can last weeks the next time, or months, or if you're really unfortunate years, then go into remission for years...or not.
I used to have similar, just the odd outbreak and like you I went on an obsessive hunt to eliminate the culprits including elimination diets
For some people there is an obvious allergen or trigger and for others there's a trigger they can never identify.
There's another group which is purely autoimmune and cannot be really solved with diets etc. The immune system is attacking your body, either your own mast cells or something else
Mine started as relatively short benign episodes easily eliminated with an antihistamine or two. Mine is autoimmune exacerbated with external triggers which are temperature changes, pressure, dust mite and exercise
I now have the full battalion of antihistamine plus epi-pens
I didn't take it seriously in the early days when it was easier to control even when I got a diagnosis. So I would dump my antihistamine when it appeared to disappear. I think that contributed to my immune system just getting more and more sensitive and in hindsight I should have continued taking them at a lower dose even during absent episodes
Low iron and low vitamin D are aggravators but not causes, it can be difficult to tell the difference between an aggravator and a true allergy etc. Definitely take a good quality D3 with K2 and an iron tablet with cofactors like zinc, B vits, VIT C and Magnesium if you're not allergic also take flaxseed or cod liver oil. Quercetin is something I highly recommend and you can take daily.
You should request a referral to allergy department for a prick skin test to airborne allergies. It is worth trying to eliminate food intolerances or allergens as they can be intermittent triggers, but usually the NHS won't test for these in adults, as they say they are not the cause of it.
There's also a strong association with thyroid illness, so get yourself checked for thyroid hormone levels including the TPO antibodies. Many people can have them without knowing. Again unfortunately you may have to get the antibody test privately. Your GP can definitely check your thyroid levels though.
I hope this calms down and you get back into control of it. But definitely be alert now and try to reduce your inflammation load.