Does a skin reaction to milk warrant a further investigation into an allergy to ingested dairy products?

I seriously doubt this has to with milk. It's entirely possible there was zero milk in the rinse. Have you contacted the spa to find out what the rinse was? But in answer to your question, sometimes. There are people who are allergic to, say, mangos, who can cut and prepare mangos with no ill effects whatsoever, and there are others who will break out in hives. But I'd find out what was splashed on my face, first. It's possible you have suddenly developed a serious allergy to milk. It's more like there were strong astringents or something funky in the "milk rinse."

If you've never had a problem drinking milk before, it seems unlikely that your issues at the spa are the result of an allergy to milk. What you describe sounds like an allergic reaction, but I'd tend to doubt it was the milk.

Sorry, should have clarified that I knew the difference between lactose intolerance and dietary allergies, but don't know much about immune-system-mediated reactions to milk. As in, would the lining of the gut behave the same way as one's skin? Could an allergy have symptoms that mimic intolerance? Agreed I should ask what's in it - I don't know Korean but maybe if I called someone at the front desk could help.

Well, milk contains lactic acid, doesn't it? Lactic acid is a commonly used skincare agent because it speeds up cell turnover and aids in sloughing off dead skin cells. Maybe the milk that this spa used had a concentrated level of additional lactic acid added to the milk concoction and that irritated your skin? IANAD, I am just a kid who likes skincare products, and I've got sensitive skin and some dietary sensitivities too.

Follow up question: do you remember if the milk was like milk-milk or did it smell kinda sour? If it was more sour smelling, it might have been like a buttermilk or something and that usually seems to have higher concentrations of lactic acid. I think true food allergies have to do with ingestion and exposure (ie peanut allergy), not contact exposure.

Latex allergy, for instance, is a contact allergy. There are plenty of contact allergies.

I have both a contact reaction to milk products and a dairy allergy. You should talk to a doctor and get allergy testing. If you don't have to do it eliminating all dairy is really tough.

If you had a dietary allergy to dairy and you still ate a lot of it, you would know by other means, guaranteed.