If I seem obsessed with seasonal allergies, it is because I am suffering this year more than most. I am continually surprised by the odd pains I experience as side effects of my "hay fever," like pains in my molars. Let's start at the beginning.
The human body tries to eliminate those pesky allergens with mucus. Yes, the mucus really wants to be your friend, an ally in this battle; but if the mucus doesn't flow out, when you try to blow it out, it hangs around in your sinuses creating congestion and causing pressure and pain. We have many sinus cavities in our skulls, but the ones on either side of your nose (aka the maxillary sinuses) need to drain upward, an action not so easily accomplished if we are upright, which we are for most of the day. These sinuses sit on your upper jaw, and when congested they can put pressure on the surrounding areas causing pain in your upper teeth, cheeks, and right below the eyes. If your sinuses become inflamed or infected, the pain could be even more extreme. Not only do those allergens make your eyes and nose run, they can indirectly create tooth pain.
If you are experiencing dental pain and are prone to allergies accompanied by nasal congestion and sinus infections, then you can probably assume that sinus pressure is causing the pain in your teeth. If you're not so sure, give your dentist a call.