The one thing I know about hives is that it is very hard to figure out what has caused them, aside from obvious signs (which you have excluded very thoroughly). I had something similar, short term occur not long ago (but not on me), and I wracked my brain to associate it with something specific. In the end, I could not. The hives luckily went away after a couple of days, but I never found out what it was.
I have never heard of something like that being associated with breastfeeding or hormones (not that it couldn't happen). However, from what you describe, I would hazard a guess that your body was stressed and it was a breaking point that your body was exposed to that lead to the outbreak. You mentioned being stressed beforehand. You don't need a specific trigger, but the tiny bit of stress caused by the full night without nursing could have been a trigger. Or, it could have nothing to do with it. Or, somewhere in the middle, it could have been a minor stress on your body, on top of all the other stress, that lead to the outbreak.
In my gut I don't feel it is hormone-related and that it is stress related. Has it abated at all? Have you tried to de-stress (which of course is hard, because being stressed about the hives doesn't help them go away if that is the cause...)? Could there just be a heck of a lot on your mind affecting your body... including ttc a third child?
These are all just ideas, but since I know the feeling (I have also had hives in the past, once I suspected what caused them, once I never found out), I thought the ideas might help.
How about exercise, fresh air, and distraction? Have you tried to incorporate this?
I hope it gets better and slowly goes away. I hope your stress also ebbs away with time.
ETA: stress and hormones affect one another, too, and so it occurred to me that it could be both interacting.
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