Although many allergies start in childhood, it is entirely possible to develop a new allergy as an adult, even to something you have eaten or been around for years without trouble. Adult-onset allergies can include hay fever, food allergies such as to shellfish or nuts, reactions to medicines, and allergies to insect stings. The exact reasons are not fully understood but may involve changes in the immune system, new exposures, moving to a different environment, or other factors.
New adult food allergies in particular can be significant because they sometimes present with more severe reactions. Some adults also develop pollen-food syndrome, where eating certain raw fruits or vegetables causes an itchy mouth in people who already have pollen allergy.
If you notice new allergy-type symptoms, keep a note of what seems to trigger them and how quickly they come on. See a doctor for assessment rather than self-diagnosing, especially if a food or medicine seems involved, so the trigger can be confirmed and you can be advised on avoidance and emergency treatment if needed. Seek urgent care for any reaction with breathing difficulty or swelling.