Is It Safe To Take Hifenac-P For Severe Muscle Pain In Shoulder And Back?

Hi. I am having severe muscle pain since last 4-5 days. the pain started from my right shoulder and back 5 days before which then moved to left side of the back 3 days ago and then towards lower back along the spinal cord. I am not able to stand complete straight or sleep straight on my back. I have taken Hifenac-P twice yesterday but still having some mild pain. One of my friends who also had same problem suggested me to take Flexura-D. Can you plz suggest- which one should be more effective- Hifenac-P or Flexura-D?