Brief Answer:
Most probable causes explained

Detailed Answer:
Hello dear Amydef,

Thank you for using our site to post your question,

Flat stool may be cause by various factors.
-- It may be normal, depending on the diet - the diet which causes constipation, because the stool is firm, sometimes it takes the different shapes. This should not happen often though, because otherwise it may be a problem.
-- Irritable bowel syndrome. It is unsure what exactly causes irritable bowel syndrome but most probably it is because the food is forced to go through the intestines either more quickly or more slowly than normal. You should know if you have this problem, because its a chronic disease that starts generally in early ages.
-- A polyp, or a mass in the bowel that makes the bowel content stay more in the bowel so as a result be more firm, and also since it passes though a smaller, flat diameter of the bowel, it takes the shape of bowel that polyp creates in that level.

All of these conditions are accompanied by other symptoms which are bothersome.
A colonoscopy would for sure rule our a cause, if it is a pathological one.
Meanwhile, take care to eat a healthy diet and drink plenty of fluids.

Hope this is helpful.
Let me know if you need more clarifications.

Take care.

---