The symptoms as mentioned in your post may be related to the fatigue
which occurred due to 18km walk.
If it is your body's protective mechanism to flush out the toxins, the symptoms will subside on its own in 2-3 days.
However, if the symptoms still persist or aggravate, it may indicate Gastroenteritis
, which is a reason for concern.
Under such circumstances, it will be better to consult a Gastroenterologist
for further management.
In the meanwhile, take proper rest, maintain adequate hydration in the form of fluids & ORS
, and have a soft, non spicy diet.
Wishing you a Good Health.
Let me know if I need to clarify further.