Side sleeping on left stomach pain. Not!

I am facing stomach pain while sleeping on the left since last two days. Initially thought any gastric issue. But after a revealed pain is coming only when I am sleeping on the left for some time.. throughout the day any sudden jerk that creates due to getting up from chair after a ling seating or similar caused tightness and momentary pain on the lower ribcage area of the front side...but then it goes away.. my orthopedist gave Hifenac. After taking it is good, pain was not coming due to sudden movements also applied hot water fomenting. First night was fine. Only slept on the back position.. 2nd night due to absence of pain forgot it and perhaps slept on left side for a while, some sensation of pain came back...due to hifenac dose it was not severe... immediately applied hot pad on the back rib cage, the front side tightness faded away.. Can any one tell me why it is happening.. very strange symptoms.. I first thought on day one that I have severe stomach infection but its not