I call it the true hip flexor stretch as I want you to truly work on stretching the hip flexor and not just torque your body into hip and lumbar extension.  It’s very easy for the body to take the path of least resistance when stretching.  People with tight hip flexors and poor hip extension often just end up compensating and either hyperextend their low back or stress the anterior capsule of the hip joint.
Or anything else. Pain is a poor indicator, period! The human nervous system is really terrible about this: it routinely produces false alarms, and alarms that are much too loud. See Pain is Weird: Pain science reveals a volatile, misleading sensation that is often more than just a symptom, and sometimes worse than whatever started it. BACK TO TEXT
A sedentary lifestyle can lead to having weak and tight hip flexors as they are always in the shortened position. Tight hip flexors can lead to a limited range of motion, poor posture, lower back and hip pain, and even injuries. These muscles need to get a workout when you are standing and doing movements such as raising your leg to climb stairs, run, or ride a bicycle.​