Over the past couple weeks I've been having what seem to be hives, which may be from some sort of allergic reaction as I've made drastic changes to my eating habits in the last month. But from what I understand of hives (having never experienced any sort of allergy before) they tend to appear and disappear within a few hours. Mine start out in the morning as 1/2" sharply edged raised areas showing the usual blanching like mosquito bites (minus the little indent in the middle and slightly more irregular in shape, like little outlines of state maps), but they're very consistent in their gradual transition into softer-edged, 2" red discs over about 12 hours. Within 24-48 hours all trace of them is gone, but by then they're generally replaced by fresher welts, 3-4 of them almost every morning.
Do you think they could be insect bites? I've been keeping my eyes peeled and haven't found any hint of bugs in my apartment beside the occasional fruit fly on my garbage, and I don't seem to get these at any time other than when I'm asleep. I'm just trying to narrow down the possibilities, I've never dealt with something like this before and google is giving me way too many avenues to look down.