For the past five or so years I have had a recurring injury to my hip: the pain originates pretty much at the front of the thigh exactly where the torso meets the legs: I believe it is a hip flexor muscle.
This injury has gone off and on. Whenever I start to get some very good training it seems to come back. As the years have gone it comes back more quickly each time, after returning from each running hiatus.
Some of my hiatuses have been quite long - 2+ months, at least, and I do believe some have been much longer. The pain goes completely away, until my running intensity picks up again.
Because of this, I don't think I could have actually torn anything significantly - what I am thinking is that there is scar tissue from an original injury that has never quite healed, and keeps getting aggravated.
There could be issues with the flexor, or maybe the bursa. Watch this video for a test to see...
There can be other stuff wrong with the joint or joint capsule, but sometimes these things look like causes and don't pan out. MRIs of the hip labrum, for example, will often show a tear that is blamed for certain patterns of pain, depending on the location. If the tesr interferes with the muscle or nerve, that is a distinct possibility, but often tears are found in people without pain, meaning tear=pain is not always true.
The main hip flexor (iliopsoas or psoas) is mostly buried beneath your guts, just peeking out enough on the top of your thigh to be felt, if slightly. It clusters around the lumbar spine, where it gets its support. There are techniques for accessing and treating it, but due to the sensitivity of the location, is difficult to treat. Here is a video demonstration of Dr Ben Benjamin finding and treating the psoas...
Be aware that many muscles assist the psoas in hip flexion, including several adductor muscles and the rectus abdominus (sixpak) muscle.