I finally discovered at least one trigger for my migraines...Chinese Privet.  These are always located in the sinuses...behind/near the eyes.

What's odd is that Zyrtec doesn't prevent them but Benadryl seems to help...both should be antihistamines.

Also, if I breathe through my mouth and not my nose as soon as I detect the scent, I don't have a reaction.  Isn't the allergen still getting into my body?

I don't (often) get migraines, but have had a lifetime of significant inhalant allergies and the sinus pain that comes with them. Not all OTC antihistamines are the same. I have tried all of them.

Benadryl (Diphenhydramine) works great but is also a potent soporific, which is less than ideal.
I tried Zyrtec (Cetirizine) and 100% of the time it made the symptoms much worse for about 36 hours. I'd rather be sleepy. Oh, it also makes you sleepy.
I ended up with Claritin (Loratidine). It doesn't do anything to treat or block pain, but does block histaminic reaction to the point of not getting the inflammation that causes the pain. Not sleepy-making either.

$0.02
I am not a doctor, YMMV, etc.

Gave Claritin (generic) a try...it seems to help more than others.  Thanks for the suggestion.