Reaction to hot tub. The chemicals in hot tub water are too small to be recognized by the immune system so they don't cause allergies but if their levels are too high they can irritate the skin. Also, a rash after being in a hot tub may be a skin infection from contamination in the water which should clear by itself. If your skin doesn't clear in 7 days, contact an md.
Possibly. All chemicals have the potential to be at least an irritant to the skin some can cause allergic contact dermatitis. Patch testing can help understand is there is an irritant or allergic component.