A warm, raised, itchy spot on your skin is most likely to be a hive (urticaria is the official medical term). Hives
are blotchy areas of skin inflammation that are caused when certain cells in the skin are stimulated to release allergy - related chemicals, leading to inflammation and itching. The spots tend to come and go over a period of hours to days and then clear up, assuming the offending agent is removed. In susceptible people, hives can be caused by physical triggers like heat or sweating or rubbing of the skin (all things that may have happened in a spa). Perhaps more likely, exposure to an offending chemical (such as a lotion or personal care product) may have done it.
Another possibility would be a worsening of eczema
, dry skin, which can occur if you are susceptible to this (if you have patches of dry skin at baseline), upon exposure to skin irritation and drying (such as might occur with spa skin treatments).
If the area of redness becomes painful, very warm, or if the area of redness starts to spread out, then these might be signs that this is actually a skin infection, in which case you would need to see your doctor