So maybe you've ruled out coming into contact with an allergen like rubber or perfumes.
But what if it's not something outside your body, but inside? Could it be something you've ingested?
Certain people can have negative reactions when starting a new medication. You can develop what's called a drug rash, either as a side effect or as an allergic reaction to the medicine [source: Mayo Clinic].
It could happen with any medication, but common types are antibiotics, diuretics or anti-seizure prescriptions. And in some cases, sunlight can intensify the effect.
Drug rashes often start out as small red dots, spreading to larger areas [source: Mayo Clinic].