17 nd back pain! help please??

i have a pain in my left lower back/side area.. it usually only hurts when i for example, sit on the floor and try to extend either my left leg flat on the ground or both legs flat at the same time. it also hurts when i walk sometimes.. when i would put my left leg forward to far.. and if i stand with my feet together or apart and bend over w/o bending knees. i went to hospital awile ago for it and if i remember right they said was a muscle sprain..?  but it has not gone away since then..  it was really bad when it first started.. it is not as bad now but it still hurts.  any ideas of what it could be from..or what i should do???