Rover:
I had pain off and on for 20 years before getting rid of it, from a severe groin pull. A training shoe specialist told me what I was doing wrong.
Every step you take, the hip flexors are engaged to bring your thigh forward (or inward). So if its injured, you keep it injured unless you rest it. Oops !
If using heat to increase circulation, you might be overdoing it. You want to increase circulation, not break down the muscle with heat and serve it nice and tender for dinner. LOW HEAT. And alternate with an ice pack every 20 minutes.
Next, use a large ace bandage to wrap the upper thigh and support it around your waist. You do this to reduce the strain on the flexors. The stretched bandage will give them some relief. Put your foot on a low step, or put he injured leg slightly forward, and wrap so the bandage is stretched in this position, and wrap around the thigh and pulled up around your waist. You can alternate lifting from inside or outside. You'l' know when you've done it right.
Causes of continuing weakness.
You need to strengthen the muscles once healed, and you can also reduce the load while rehabbing the muscles. What ? Yes, reduce the load. You reduce the load by shortening the lever that is being swung forward by the muscles. That means lifting your foot higher, when the leg is BEHIND you. Not a flexor lift of the thigh, but a hamstring lift of the foot. When you pick up your foot, swing it up at least an extra couple inches.
Re-aggravation of this injury, for me, came from being unaware I was not raising my foot high enough behind me, flexing the hamstring, when running. The foot would be lower, the flexor had greater weight to move, and of course, I re-injured the muscles.
So become extremely conscientious about kicking up both feet behind you when running, before you swing the leg forward. If running and you feel the flexors developing pain, you will get IMMEDIATE relief by kicking up the heel behind you much higher before swinging your leg forward.
What you may need to do is strengthen the hamstrings more, so this becomes natural. Then the flexor will never get an increased load, unless you forget or become lazy and drop the foot too low while swinging your thigh forward.