As the gallbladder is on the right side it might be that gallstones were an incidental finding on the ultrasound and not linked to your pain. Pancreas and stomach are on the left where you say your pain is and can be visualised on ultrasound. The transverse colon also sits below the ribs on the left so my guess would be that the pancreas and stomach were fine, the gallstones wouldn't account for left sided pain, so the doctor ordered a fit test to see if there was any problem in your colon (fit test looks for traces of blood which can be from piles, straining etc). If there was a problem there this may account for the pain, also the the colon can't easily be visualised using ultrasound. 
 

Only a small proportion of positive fit tests show anything sinister so try not to worry as there are numerous reasons for pain that aren't cancer. Positive fit test and other risk factors such as age always warrant an urgent referral which in itself makes you panic but the number of urgent referrals that find cancer is ridiculously small. Try not to worry too much. Good luck.

Thank you Lilly. The way you explained it makes sense xx