My lower back started hurting constantly without really injuring it. The only thing I have done recently is sit on an airplane for about 20 hours and drive for eleven. It is tender to the touch on the lower left side. The pain is all the way across the lower back.
I would suggest you to go with hot fomentation and painkiller like diclofenac with sanitizing twice daily for few days. Once the pain subsides go with back strengthening exercises. Hope this helps.