Musculoskeletal strain or lumbar degenerative disease: Given the patient's age and the presence of point tenderness to the right paraspinal L5 region without evidence of neurological deficits (negative straight leg raise), a musculoskeletal issue such as a strain or degenerative changes in the lumbar spine is the most plausible explanation for her symptoms.

Kidney stone: Although the urine is negative for blood, it's essential to consider kidney stones given the patient's history. Sometimes, kidney stones may not cause hematuria (blood in urine), and the pain can be severe and localized to the flank or back.
Constipation or bowel obstruction: Despite the patient having a bowel movement after an enema, persistent or recurrent constipation could contribute to back pain, especially if there's a significant amount of stool in the bowel that's causing pressure on the spine or surrounding structures.

Abdominal aortic aneurysm (AAA): Although the patient does not have a pulsatile abdominal mass, AAA is a critical diagnosis to consider due to its high mortality rate if ruptured. The absence of a pulsatile mass does not entirely rule out AAA, especially if it's small or not significantly dilated.
Spinal infection or osteomyelitis: Infections of the spine can present with back pain and point tenderness, and given the patient's age, her immune status might be compromised, making her more susceptible to infections.
Cauda equina syndrome: This is a neurological emergency that can present with low back pain, among other symptoms like urinary retention or incontinence, and neurological deficits in the lower limbs. Although the straight leg raise is negative, this diagnosis should be considered if there are any red flags such as recent onset of urinary or fecal incontinence.

Spinal tumor: Although rare, spinal tumors (either primary or metastatic) can cause back pain and point tenderness. Given the patient's age, the possibility of metastatic disease from another primary site should be considered.
Osteoporotic compression fracture: This could be a cause of acute back pain, especially in an elderly patient with osteoporosis, although typically, there would be a history of trauma or a sudden increase in pain.