A few things that would be rather common culprits:
aniamls (yes, even pets that you once tolerated, if you have been living cat/dog free at school)
air fresheners/room sprays
a source of mold in the house
laundry/personal products (soap, shampoo, laundry detergent/softener, toilet paper, etc.)
It's also possible for hives to be viral or from seriously irritated skin if you've moved suddenly from a more humid environment to a VERY dry one, or vice versa.
ETA: looks like "overnight" is when you began having the problem. Pets on the bed? A mattress cover/pad that contains an allergen? A pillow that does? Linens on the bed?