I finally went for an xray, to determine if there was a kidney stone.
There wasn't. The doctor referred me back to my usual family physician, presumably because he has my MS history and knows more.
Now the diagnosis is a sprained lower left back. With the heat therapy and prescription Naprosin, I am finally getting some relief from the pain. The expensive antibiotic did clear up the leukocytes and blood in the urine, so I think I will save it for later. UTI's are a constant threat for me.
For now, it's pain killers, heat stick-ons (which work very well, thank you very much), and physical therapy will begin three times a week, starting Wednesday. It's at the end of the day, when I am shakiest, so it should be interesting to observe. Let's hope it's not painful to accomplish.