I'm sorry to hear your little Corgi is having problems with her leg.
Being an active girl, and being an older girl, there are a couple of major concerns here. I am curious as to where this swelling is occurring first of all. If indeed the entire back leg (as short as it may be in a Corgi) is 'rock hard', there is a chance of there having been some internal bleeding going on. Thus, Joan strongly suggested stopping the motrin, which I completely agree with. Motrin is not safe for dogs, even in low doses. And any aspirin-type medication can slow bleeding time, which can hurt this problem, not help.
Second, with an active dog, injuries can occur to the joints which can cause some severe swelling within the tissues, which only with proper medicating from the vet (often SAFE anti-inflammatory meds along with pain medications) and strict exercise restriction, can this heal properly. As you'd like to see how this does at home, although not the best idea, I strongly suggest then to keep your dog strictly confined except for short leash walks to do her business. Try cold dry compresses to the swollen area 3-4 times a day for 5 minutes at a time. Even when improvement can be seen, continue the exercise restriction. Even sprains, though they do not present this way, take time to heal. Fractures take longer, and tears of ligaments and more severe fractures will not heal on ther own and without proper treatment lead to severe arthritis.
Another concern if this occurred quite rapidly is a bone tumor or a tick borne illness causing fluid build up in the leg (common in many areas of the U.S.) This is not something you'd be able to try treating nor figuring out from home, but either can become deadly.
My best suggestion is to follow Joan's above suggestion and seek a vet. I certainly understand your hesitation and in your best interest I would seek a different vet than you've seen in the past. And we are always here for you to help you understand what the vet is saying. Decisions do not need to be made asap when suggestions are given, and a vet cannot treat a certain way without your approval. YOU are essentially in charge of your pet's care, and you can choose to only accept treatment on what you are comfortable with. But indeed your dog's symptoms are quite severe and not the norm for just a run-of-the-mill sprain which can heal in a day or two.
I wish you and your dog nothing but the best, ***** ***** he's back to his normal self as quickly as possible. Don't hesitate to reply if there is anything myself or Joan can help you with.