What Causes Lower Backache On The Left Side?

My symptoms include...three episodes of extreme lower back pain, left side; onset late in afternoon but each time subsided by next day. Nausea, frequent urge to urinate, but no fever. The three pain episodes have been within the last 7 days. Kidney stones? UTI? Am waiting for my doc to return call. Any thoughts on this come and go situation? The episodes leave me pretty wiped out the following day.