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In an 11 year old dog, there are a multitude of things that could be going on from the benign with something like arthritis to something more serious such as cancers.
I don't think this is something you need to rush to the emergency vet for. But I would absolutely give the vet a call first thing Monday morning and have them take a peek at her. They'll do a physical exam on her leg and may want to do an x-ray or run some blood work, just to make sure that there's not something more serious going on than just a little geriatric aging change like arthritis.
Regardless the cause, the vet will be able to get her on some medication to bring down the swelling and something to help get her pain under control so that she's more likely to want to use the doggy door and steps.
If she gets to a point this weekend where whe absolutely can't bear weight on that leg, then you should take her in to see the emergency vet, but I suspect she'll be okay until Monday morning.
I hope this helps.
With that new information, it sounds like she's got what is called edema in that leg. I think the best bet is for you to take her to a vet tonight. Edema in the extremities can be an indication that there's a heart problem going on, so getting her treatment tonight would be the best thing for her.