What Causes Dark Stinky Stool?

My daughter, now over 3 1/2 years old, has always had light yellow brown stool that is very stinky and on the loose side. It smells a bit like bleach at times. She has tested negative for dairy, corn and wheat allergies. She likes to put everything in her mouth and licks and chews things all the time. She is impulsive and hyperactive often. She eats probiotic yogurt, still nurses at night, and was delivered via cessarian section. I know that fecal implants have been helping people restore their intestinal flora and regain health these days. Intestinal health being do tied to mental health, I am wondering if this could be an imbalance in her gut. What could we test for?