Could she have been bitten by a spider, bee or other insect (depending on how cold it is where you are located? A spider bite could cause an area of pronounced inflammation and a spider bite can occur year round, in all climates.
If it is a spider bite it would require antihistamines, steroids, and possibly antibiotics as well as supportive therapy if necessary. It doesn't sound as if it is necessary in her case, thank goodness.
We've had a couple of dogs bitten by copperheads that got enormous swelling of their heads and snouts (thats usually where they are bitten).
The only treatment they needed was antibiotics and antihistamines, and all recovered uneventfully.
Depending on your location (obviously, snakes are not a problem in most areas right now), it could have been that.
Thanks for your input Peekawho! I wish I could tell everyone who is posting a question to list their location.