Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl has had a couple episodes of soft tissue swelling of her feet, and fever.
If it was the same foot I would think that this could be related to a localized bacterial infection, possibly related to a foreign body (plant awn, sliver) trapped in the soft tissue of her foot. That would explain her good response to anti-inflammatories and antibiotics. But then I would not expect this to affect another leg.
The other possibility is Lyme disease, as that can affect different legs, and if not treated with the correct antibiotic for a long enough period could reflare. However usually Lyme disease causes joint swelling rather than soft tissue swelling. To be thorough I would recommend testing her for tick borne diseases.
Perhaps this is related to an allergic reaction, which would explain why your veterinarian prescribed an antihistamine (Benadryl) and a steroid (prednisolone).
It is also possible though that this is related to some sort of primary autoimmune mediated response (body attacks itself), such as idiopathic polyarthritis or systemic lupus. These won't cause changes that we can see radiographically, which would explain her normal radiographs, and would respond to anti-inflammatory doses of steroids.
At this point since she has started steroids it may be more difficult to get a diagnosis, but to rule out infectious causes or metabolic disease leading to tissue swelling I would recommend a complete blood count, biochemistry profile and urinalysis along with a tick disease panel.
Please let me know if you have any further questions.