A 33-year-old female complains of escalating spastic pain in the abdomen after the psycho-emotional stress. The patient has intermittent bowel movements, that is 2-3 bowel movements after waking up alternate with constipation lasting for 1-2 days. Objectively: body weight is unchanged, there is moderate pain on palpation of the sigmoid colon. Hb- 130 g/l, WBC- 5,2 • 109/l, ESR- 9 mm/h. Proctosigmoidoscopy causes pain due to spastic bowel condition, intestinal mucosa is not changed. In the lumen there is a lot of mucus. What is the most likely diagnosis?