Have you taken any anti-histamines before?
If so, take the same one for two days running, and see if it subsides. If it does, it is an allergy of some sort. At that point, it’s just a matter of eliminating foods until you find out which one.
I had hives in the second week of Keto. Thought it was Keto Rash, but it turns out it was the switch to erythritol as a sweetener. It’s a sugar alcohol, and apparently although rare in all populations, is slightly more common in Asians. (I’m Chinese descent).
It’s interesting, because most things I am allergic to, I react either immediately, or at most, hours after exposure. It took a week to build up, and 2 days to subside after I stopped taking it. It seems to need to build up, as excretion of sugar alcohols is slow. Apparently about 75-80% in a 24 hour period.