There are five sensations that could signify a medical emergency.
Sharp pain instead of a dull ache:

This could signal a torn muscle or ligament, as well as a problem with an internal organ in the back or side.
Radiating pain:
If the discomfort “moves” or “shoots” to the glutes or legs, it could be a sign of nerve compression.

Radiating pain may indicate nerve injury.
Leg weakness:
Compressed nerves in the spine can induce limb weakness due to disorders such as sciatica or spinal stenosis.

Sudden leg weakness, on the other hand, could signal a stroke.
Incontinence:

Back pain combined with inability to regulate bowels or bladder could indicate significant nerve compression or a spinal infection like discitis or meningitis.
Numbness or pins and needles in the groin or glutes:
This is referred to as saddle anesthesia and can indicate a major nerve or spine problem.

If you suffer leg weakness, incontinence, and numbness all at the same time, you may have cauda equina syndrome, a dangerous disorder marked by nerve injury to the spinal cord.

This is a medical emergency, and patients will almost always require rapid surgery to relieve the pressure on the nerves and prevent irreparable harm.