It sounds to me like a bee sting or stings. For something to get that swollen in 20 minutes it has to be some type of insect sting. Where do you live (general area)? Do they have bark scorpions where you live? That's another possibility. Benadryl is excellent for relieving allergic reactions such as swelling from a bee sting. Please let us know how she is doing.
Ghilly
Please excuse all my typing mistakes. I'm struggling with a new keyboard that won't do as it's told
Oh boy! I'm so sorry, I have only just come online and seen your post, so I can tell it's quite a few hours since you posed this. I hope your dog is OK.
If ever in serious doubt, the very best thing is just to dash to the vet, and not wait here.
nyway, it could have been a bee or wasp sting perhaps? If dogs snap at them, which they unfortunately do sometimes, then that's it, -they get stung. If that's the case with her, then it's better to be stung on the lips (painful though it must be) than to be stung in the throat, where swelling could be extremely dangerous, life-threatening in fact.
As a first-aid measure I think you probably did the right thing, cooling the area would help to reduce any swelling and ease the pain.
How is she now?