i think it's possible to develop allergies --- either that or a pre-existing allergy which never caused any problems ever before may suddenly flare up
also, bear in mind that an allergy can be complicated by other factors --- so, say, you have a manageable allergy to something, a no big deal allergy. but suddenly, it gets real dusty or moldy, or you end up in a house full of smokers or something, and those irritants can make the allergy seem to go nuts.
i suddenly developed an allergy to walnuts the last year or so. it's very distressing because i love walnuts. but out of the blue, they began making my mouth itch and break out.
maybe the same has happened with you and doggies?
i don't know the solution. i'm allergic to cats and grew up with them --- never remembered one single problem with them either. but after i'd been away from cats for a year or so, boy, did i ever notice! i love cats but can't stand to be around them because they make my face itch if i get anywhere near them.
cat allergies seem to be worse than dog allergies.