I've got yellow stools and discomfort in lower back sometimes when lying down what is this?

Bile-colored stool: The amount, color, frequency of stool varies with foods, speed of emptying of the GI tract, medications & supplements ingested, gut flora (unique for each individual), additives (like blood), subtractions (like absent bile), infection, inflammation, & other factors. If stool pattern has changed, is unusual for you, & the change persists, or is accompanied by other symptoms, see your doctor.