I am sorry your girl is having trouble.
The sensitivity to light can be due to various reasons and allergy flair ups can be one of them. This is not a typical allergic reaction which adds to your vets confusion I am sure.
With the Benadryl and steroids working and the problem coming and going then allergies would be the most likely cause however without some additional tests it would be impossible to say for sure what is going on.
Other possible causes would be Infection, Elevated blood pressure, ulcers just to name a few.
Here is a list of more possible causes of light sensitivity
Horners syndrome is also possible but this does not normally come and go.
Because this keeps happening it may be best to have her seen by a eye specialist. I would have her re examined by your vet tomorrow if this is still a problem.
As for the prednisone this may help much more then the Benadryl and as long as they are not on it more then a few days we normally do not have to wean them off.
If you have sterile saline used to clean contact lenses you can flush the eyes and see if this helps.
I hope all goes well and they find the cause soon.Please reply back if you have additional questions or concerns