You could test to see if something in your bedding is causing your hives.
You could have an allergy to cotton, the dyes or chemicals in the sheets and
blankets, and/or to dust mites.

You might consider sleeping in a non-cloth, non-upholstered recliner or
couch one night, without a blanket or a sheet. If you wake up in the morning
without hives, then you know that it is one or more things in your bedding
that is causing your hives. You could then sleep on that same recliner, but
use a sheet to cover yourself, and see if you have hives in the morning.

Since your hives clear up after you get out of bed, your bed is very suspect
to me.