Diverticulitis: Given the patient's age, symptoms of generalized abdominal pain that is worse in the lower abdomen, nausea, and changes in bowel habits without vomiting, diverticulitis is a strong consideration. The elevated CRP (78) supports an inflammatory process. The patient's past surgical history of appendicectomy does not directly influence this diagnosis but rules out appendicitis as a primary concern.

Gastroenteritis: The symptoms of abdominal pain, nausea, and increased frequency of small-volume stools could suggest a gastrointestinal infection. However, the absence of vomiting and the presence of pain on urination might lean the diagnosis away from a pure gastrointestinal cause.
Urinary Tract Infection (UTI) or Prostatitis: The pain on urination could indicate a UTI or prostatitis, especially given the patient's gender and age. The normal urine analysis results do not strongly support this diagnosis, but it cannot be entirely ruled out without further testing.
Inflammatory Bowel Disease (IBD): Although less likely given the acute onset, IBD (such as Crohn's disease or ulcerative colitis) could present with abdominal pain, changes in bowel habits, and systemic inflammation (elevated CRP).

Appendicitis: Despite the patient's history of appendicectomy, it's crucial to consider the possibility of a recurrent appendicitis or a stump appendicitis (if the appendiceal stump was left during the previous surgery). The symptoms and elevated CRP warrant careful consideration of this diagnosis.
Intestinal Obstruction: Although the X-ray did not show clear evidence of obstruction, and the patient has not vomited, partial or early obstruction could present with similar symptoms. This diagnosis is critical to identify promptly due to the risk of bowel ischemia.
Ectopic Kidney Stone or Bladder Stone: Although the patient denies a history of kidney or bladder stones, it's essential to consider the possibility of a newly formed stone, especially given the pain on urination.

Mesenteric Lymphadenitis: This condition, often mimicking appendicitis, could be considered if other diagnoses are ruled out. It typically presents with abdominal pain and might be associated with systemic symptoms.
Meckel's Diverticulitis: A rare condition that could present with abdominal pain, nausea, and changes in bowel habits. It's usually associated with a history of similar episodes or gastrointestinal bleeding.
Abdominal Aortic Aneurysm: Although rare in this age group, an abdominal aortic aneurysm could present with abdominal pain and should be considered, especially if the pain is severe and associated with back pain or a palpable abdominal mass.