What Causes Left Sided Pain In Pelvic Region?

I am a 40 year old female with two healthy sons. I am having pain on left side of my waist in the bone right there. I have no problems in walking or any movement but as soon as I come to bed during day time or night it starts hurting . I have hypothoriod I am on medication for the same all reports are normal. Please suggest me what is this and which specialist I should consult?