I agree wholeheartedly with you that this isn't likely to be an orthopedic issue.
This looks like either edema (fluid retention) or possibly cellulitus (infection along tissue planes). At this point, it may be difficult to tell the difference.
1. The "sore" may have been a source for infection which has spread rapidly which sometimes happens. Some patients will run fevers so if you have a thermometer, it may be helpful to know if her temperature is high or not; normal is between 100 and 102.5.
If this is infection, then she'll need antibiotics before improvement is seen. If you have access to any drugs of this sort, please let me know and I can provide a dose for a dog this size.
2. If this is edema, the skin may "pit" or remain slightly depressed when you press it firmly in one place. The underlying causes for pitting edema range from cardiovascular issues to liver problems to impaired flow of the lymphatic system.
This type of problem often takes quite a bit of testing to determine the underlying cause.
It might be prudent to have her seen today unless you have antibiotics which can be given. Hopefully, this is just an infection but since it could be something a little more serious, I probably would want to have her seen sooner rather than later.
I hope this helps. Deb