I Am Experiencing A Knifelike Pain In My Left Leg

I am experiencing a knifelike pain in my left leg from above the knee downward. It started in my groin and progressed over several months. It is now more intense. There is no visible outward sign , no swelling.
It hurts most when I am on my feet. periodically it clears for short time and returns as though it has been struck by a heavy weight and I have to support the leg. It also causes difficulty walking and it feels heavy.
It is confusing owing to the spasmodic subsiding of pain when sitting and the very rare time I am without pain when standing, that returns when I stand to walk a very short distance. The thigh pain keeps me awake at night.
Will you kindly advise if the symptoms offer any indication of the cause or treatment required.
Regards,
Joyce Bell