Constipation or Irritable Bowel Syndrome (IBS): Given the age and gender of the patient, and the non-specific nature of left-sided abdominal pain, constipation or IBS are common and likely causes. These conditions are prevalent in young females and can cause chronic or intermittent abdominal pain.

Ovarian Cyst: Ovarian cysts are common in women of reproductive age and can cause intermittent, often sharp, pain on the side of the cyst. They can rupture, causing severe pain, but many are asymptomatic.
Diverticulitis: Although more common in older adults, diverticulitis can occur in younger individuals, especially if they have a history of diverticula. It typically causes left lower quadrant pain.
Gastroenteritis: Inflammation of the stomach and intestines, usually due to infection, can cause abdominal pain, nausea, vomiting, and diarrhea. The pain can be diffuse or localized.

Ectopic Pregnancy: Although less likely if the patient is not sexually active or is using effective contraception, ectopic pregnancy is a life-threatening condition that must be considered in any female of reproductive age presenting with abdominal pain. It typically causes sharp, stabbing pain.
Appendicitis: While appendicitis typically causes right lower quadrant pain, it can occasionally cause left-sided pain if the appendix is located in an unusual position. It's a surgical emergency if not promptly treated.
Kidney Stone: A stone in the ureter can cause severe, colicky pain that may radiate from the flank to the groin. It's essential to consider this diagnosis due to the potential need for urgent intervention.

Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis): These are chronic conditions that cause inflammation of the gastrointestinal tract. They can present with abdominal pain, changes in bowel habits, and other systemic symptoms.
Endometriosis: This condition, where tissue similar to the lining inside the uterus grows outside of it, can cause chronic pelvic pain, heavy bleeding, and infertility. It's more commonly associated with pelvic pain but can cause abdominal pain.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood flow. It's rare in adults but can cause severe abdominal pain, vomiting, and bloody stools.