I decided not to take Benadryl and just iced this bad boy. The swelling went down overnight and is all but gone this afternoon. Turns out, we got attacked by Yellowjackets. They're a type of wasp and are more aggressive than bees, particularly in the fall. They can bite as well as sting and since they don't lose their stinger they can sting you several times and will do so unprovoked. Yellowjacket colonies are largest in late summer/early fall. If someone happens by a nest, which are usually built underground, they will swarm attack.
Uh, yeah. Good to know.