Hello and Welcome to ‘Ask A Doctor’ service.
I have reviewed your query and here is my advice.
Pain in the left upper thigh that causes trouble in sitting, feeling of spasm,
stiffness that gradually doing away indicate a muscular cause.
This can be secondary to some direct or indirect trauma or
injury while exercising or playing.
I would advise to consult an orthopedic Surgeon for clinical evaluation, an actual examination and
MRI or so for confirmed diagnosis and proper management as this is sort of a crippling problems if not cured within time.
Till then take a strict
bed rest and avoid any movements that cause pain or spasm.
Hope I have answered your query.
Let me know if I can assist you further.