Have a female cat,approx 3yrs of age with bladder frequency?

I had her to the vet Feb 8, where he did an x-ray to rule out kidney stones. He put her on clavamox 2 times a day till the med was gone, approx 13 days. After this she started to have frequency again, and blood in her urine. I took her back again today, and he feels she is just forming crystals. He put her on cefa drops 1 time a day, and suggested prescription food, which I will be trying, wet and dry. I am concerned as I see there is corn meal in the dry food. I have been feeding Eagle Pack Holistic Select, as the dry food and Fancy Feast as the wet food. Is there anything else that i could be feeding her for this condition? If she doesn't improve, he suggested a sonogram,and a urisis where they sedate her, and go into the bladder. Any other ideas anyone might have would be appreciated. She is eating, and does not seem to be in any pain.