What will happen if i took an expired store brand allergy pill?

Probably nothing: It's not good to take expired medicines, as they may have lost some potency and might not work as well. In most cases though, they are not likely to be harmful to you. For allergy pills, there should be no harm, but they might not work.

Probably nothing: Expiration simply means the medication is not considered to be 100% or maximally effective if not taken by the expiration date. But the worry for untoward side effects is miniscule. I wouldn't worry.