Why would someone all of sudden develop sensitivity/allergy to multiple different medications or environmental stimuli that were not allergens previously? what could be the underlying condition that causes this?

You're correct in looking for factors other than allergies to explain intolerance s to multiple meds. I always looked askance when a patient came in with a long list of allergies to medications. In reality it just doesn't occur. Intolerances to multiple meds are another matter. I ask for a description of the symptoms. If they're nondescript,fear from previous truly allergic reactions are possible