Hi,
Welcome to health care magic,
I have read your query and based on your question,my opinion is as follows.
Yellowish colored stools which appear seedy could bey due to bile obstruction but you have already done 
ultrasound and must have ruled out this possibility.
Also,your 
flank pain,stomach or intestinal pain with 
nausea could be related to your history of 
irritable bowel syndrome for which you must be taking symptomatic treatment. 
If you had a single episode of yellow stool then you need not to be worried as it may get resolved on its own.
If it repeats, you will require upper GI scopy along with some blood works like serum 
bilirubin evaluation, particularly conjugated bilirubin and stool examinations.
Meanwhile,Take plenty of fluids with oral rehydration salts and continue with your usual diet. Not to worry.
Take over the counter pain killers,antacids and anti-emetics medications with all other medications of your IBS.
Hope it helps.
Regards,