Allergies. Yep, dogs and cats have allergies just like we do. They can be allergic to specific foods, to bee stings, to pollen. They can be allergic to pretty much the same things that cause reactions in us.
The good news is that they react to Diphenhydramine exactly as we do. OK, that's Benadryl. Same drug; same strength but there are a few caveats.
The dosage is 1-2 mg of Benadryl per pound of pet's bodyweight. This is, obviously, a very forgiving calculation. But if you are not good with numbers or are terribly nervous, ask your pharmacist help you with the calculations for your pup or kitty when you buy the Benadryl and keep the dosage taped to the package so you don't have to do the math in real time. If you didn't do that, call your vet or one of the emergency vets. You should have their numbers in your smartphone (Safety Tip #1) but in case you don't, in Brooklyn we have:
VERG North: (718) 522-9400
Blue Pearl Brooklyn: (718) 596-0099
VERG South: (718) 677-6700
If you can't get a vet who is free in one facility, try the next. They will be happy to do the math for/with you. And they won't charge. More to the point, they'll coach you through dealing with this.
Very important also is the kind of Benadryl you buy. These days in order to get more shelf space, manufacturers mix other ingredients with their Diphenhydramine such as caffeine to counteract the drowsiness or Xylitol in the liquid to make it sweet but, as you recall, Xyletol is highly toxic to pets. So, check the label and be sure you are buying pure Benadryl. No additives.
And if pup's or kitty's symptoms don't get better in a day or two, make an appointment and see your family vet.