Help idk what's wrong w my hubby.????

He works out for the military and he also does a lot of lifting and working out. He's been having pain in his lower left/center abdomen. Went to the er & they did blood test and urine and said it all came back fine. They have no idea what it is. Is it a strain you think? It kills him to like stand or roll over in bed or like move or stretch that part of his stomach. Maybe a muscle strain?? I'm really worried about him.... please help! What do we do???