I agree that looks like hives.
Have you wormed her in the last few days? Any meds?
Have you changed hay? Even if you haven't, has it been checked for any type of weeds or mold?
If she's on a roundbale, how long has the roundbale been out?
If she's on pasture, it's entirely possible she's eaten some sort of weed that she wouldn't normally eat because grass isn't plentiful.
One of my horses comes in smelling like wild onions every night because they are growing taller & faster than anything in the pasture and he loves them.