I've noticed over the past couple of days that my left ear keeps on discharging runny, yellow and smelly stuff. Why does this happen?

Best to see MD. The ear is actually a closed system. The outer ear can't get to the middle ear due to the tympanic membrane. If there is discharge, either there is an infection , and a bad one in the canal causing it, but more likely an infected middle ear with perforation of the tympanic membrane causing fluid to go through. It is best to be examined and treated right.