I Am Feeling Mild To Moderate (varies) Pain In My

I am feeling mild to moderate (varies) pain in my left pelvis region and almost very mild in right pelvis region while doing stretching exercise. While running I do have this pain also. But during normal and fast walking, climbing the staircase and any bending I do not feel any pain.
Please let me know what it could be and should see a doctor or not and what kind of doctor should I consult..