Nicole, I would not add anything else right now to her feed until you know more of what you are dealing with. She is not eating much and adding anything is going to likely increase her inappetence. You also won't know if any side effect is from the new supplement or from whatever systemic issue she is having.
Personally, if she were mine, I would take her off of her current antibiotic and unless really contraindicated, I would have her on IV oxytetracycline. We have pockets of Potomac fever that is endemic here in Michigan. I have never had it in my immediate area, but just 30 miles west of me it is almost guaranteed if you bring in a horse from outside the area that is not vaccinated for Potomac. The signs are recognizable at the start typically with diarrhea but not always,normally a fever, and the vets here will go ahead and administer without waiting for a test, which takes several days, and you lose precious time waiting. Your vet needs to either continue to blood test and search for a diagnosis while trying treatments that are the least hazardous to your mare or needs to start consulting with professionals who can help direct the diagnosis. She can survive sole penetration and even sloughing her hoof. An unchecked systemic infection or toxin she may not.
Dawn Wagstaff and Tipperary
Saline, MI 2003