This sounds autoimmune so I highly recommend seeing your PCP to figure out who you should be referred to, ie Rheumatology, Immunology, and/or dermatology

If the spots are on the bottom of your foot, it's most likely POTS. otherwise it could be an allergic reaction to any number of things, including your own sweat (even if you never had that problem before. I developed cholinergic urticaria very suddenly when I was 21). If elevating your foot doesn't help alleviate symptoms, I would see about getting an appointment with a dermatologist

I’ve read that POTS and MCAS (Mast Cell Activation Syndrome) are often found together, and itchy hive-like patches can be part of MCAS. I have both, and I get hives/itchiness from clothes rubbing too hard against my skin, exercise, and stress, among other random things. Might be worth looking into if you have not already.