What Causes Constant Pain In Lower Left Of The Abdomen?

My husband has a pain in the left lower side of the abdomen (opposite to the apendics) which goes a little bit to the back. It's more like a sharp pain, It's a constant pain but not always sharp. He is a truck driver and the pain started a few days ago when he had to tighten the strips on the flat bed. He was constipated but drinking lots of apple juice he went the opposite. Can you please advise on what could be the cause of these symptoms. Thank you and your answer is very much appreciated