How To Treat Lower Back Pain And Muscle Cramps?

I have been having lower back pain for quite some time.however, it was on the higher side since last 2 days..and suddenly yesterday while doing work I got a severe muscle pull ...the doctor gave me an injection containing a pain killer n gastric trouble last night.though I cud sleep during night,the pain is back now... can I take hyfenac mr and how many times?