Irritable Bowel Syndrome (IBS): This condition is characterized by a combination of chronic constipation and diarrhea, abdominal pain, and changes in bowel habits without any visible signs of damage or disease in the digestive tract. The patient's symptoms and benign physical exam support this diagnosis.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic diarrhea and constipation, along with abdominal pain and changes in bowel habits. The presence of mild ileus and air-filled loops of small bowel on the abdominal x-ray could suggest an inflammatory process.
Small Intestine Bacterial Overgrowth (SIBO): This condition occurs when there is an abnormal increase in the bacteria that inhabit the small intestine, leading to bloating, gas, diarrhea, and constipation. The patient's symptoms of chronic diarrhea following constipation could be indicative of SIBO.
Gastrointestinal Infection: A viral or bacterial infection could cause a sudden change from constipation to diarrhea. However, the duration of the patient's symptoms (one to two months) makes this less likely.

Intestinal Obstruction: Although the x-ray does not show convincing evidence of obstruction, partial or intermittent obstruction could still be present, causing the patient's symptoms. This condition requires prompt diagnosis and treatment to prevent complications.
Colorectal Cancer: Although less common in younger individuals, colorectal cancer can cause changes in bowel habits, including alternating constipation and diarrhea. It is crucial to rule out this possibility, especially if other risk factors are present.
Ischemic Bowel Disease: Reduced blood flow to the intestines can cause abdominal pain, changes in bowel habits, and potentially diarrhea or constipation. This condition is more common in older adults but should not be overlooked in younger patients with risk factors for vascular disease.

Celiac Disease: An autoimmune reaction to gluten can cause diarrhea, constipation, and abdominal pain in some individuals. While less common, it should be considered, especially if the patient has other symptoms like weight loss or fatigue.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can affect bowel habits, leading to diarrhea or constipation. Although not directly related to the gastrointestinal findings on the x-ray, thyroid function tests might be considered as part of a comprehensive evaluation.
Neurological Disorders: Certain neurological conditions, such as Parkinson's disease or multiple sclerosis, can affect bowel habits. However, these would typically be accompanied by other neurological symptoms.