Poop can vary, not only from day to day, but hour to hour. Poop alone is not a good indication of illness.
A lot of things can cause runny poop - temperature fluctuations, water intake, water content of vegetables offered, worms, yeast infections, etc.
The thing to do if you are concerned is to isolate the chicken whose poop concerns you. Observe the poop over the span of that entire day. This will also give you the opportunity to observe the behavior of that individual because behavior more than any other thing will tell you if your chicken is sick. The symptoms of a sick chicken are a normally chatty and vocal individual going mute. They will be sedentary, not moving much, usually facing a wall or corner with tail held low and flat. They may nod off while standing up, not moving much from one spot. They may have no desire to eat.
The only time poop should alarm you on its own is when you see actual blood in the stool. Or worms in the pile.