Can you be allergic to something only on occasion?

I have a white German Shepherd and twice a year the dog sheds like a BANSHEE! It’s incredible. You can grab fistfuls of fur out at a time. I swear, you could make five size large adult sweaters out of all the hair she sheds during that time. Her hair is all over the house for about two weeks and it’s a never ending battle to keep it swept up.

Now, I’ve never been prone to allergies, and in all of the five years we’ve had Dakota her shedding has never affected me (except I hate vacuuming twice a day!) However, in the last week I have been doing nothing but coughing and sneezing and my nose is running and I’m pretty miserable. But….if I leave the house it goes away, so I MUST be reacting to the dog.

She’s reaching the end of her most recent shedding incident, and soon after I swept up yesterday, the itching and sneezing started receding, and it’s pretty much gone now.

How come I don’t normal have a problem, and then suddenly I DO have a problem? I mean, either I’m allergic to the dander, or I’m not, right?