My stool is a light/medium brown color, but when i wipe, it is yellowish brown. i am taking probiotics and had a cat scan and all stomach organs are normal according to my doctors! what could this be?

Yellow brown stool : I would not worry since it is mostly brown. if it were mostly yellow, it could be due to to much gluten or gluten intolerance, yellow foods like carrots/ sweet potatoes, turmeric in the diet, stress , liver , gallbladder pancreas but tests were negative or the effect of the probiotic . You could try a week to 10 days off the probiotic and then restart it and see if that makes the difference.

Normal: What you described is normal. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/.