What could be the cause of left shoulder blade pain?

For the past few days I have been experiencing left shoulder blade pain. The pain is directly under my left shoulder blade and sometimes radiates around my ribs to under my left . I have no pain when moving my arm or shoulder. The pain is relieved with heat and pain relievers but comes back again within hours. The pain has started to keep me awake at night. Does anyone have any ideas or solutions? I am a 42 year old female.