S.H.
Maybe a circulatory problem?
Not saying this about her... but Diabetics... have circulatory problems... and they do also get swelling and/or pain and tingling... of the legs and feet... but also due to water retention as well.
My Dad was Diabetic...
She should see a Specialist... but as you said, the x-rays/MRI and ultrasound, show nothing.
Or has she possibly gotten bitten by an insect???? Spiders, ants, etc.
These also can leave some bad swelling... and especially is there is an allergic reaction to the sting....
it is not always obvious... if there is an insect bite. Many people get bitten and don't even know it, for example.
Oh sorry, you said she has had this for 3 months.
Just some ideas...
All the best,
Susan