Allergies can and do develop suddenly. The human body can become suddenly intolerant to a substance and result in a reaction that us at least uncomfortable and unpleasant and at worst anaphylactic.
Treatment includes hygiene of both owner and hog. Washing your hands before and after handling will keep irritants minimal. Bathing the hog will remove allergens including saliva that produce reactions. Bedding cleanliness is key, fincard/carefresh needs regular spot cleaning with weekly clearing and wiping of surfaces, fleece changing and washing with non biological washing powder is recommended. If the wheel is washed daily this will prevent excrement and unrine falling onto your pet and footbaths minimize dirty nails and feet.
Anything can cause an allergy, mammals have dander which is a known allergen. Piriton or other antihistamine can reduce reactions and allergies with good hygiene.