severe upper backpain

I am 28 years old and just started back to college.  I had been a stylist for 9 years and always seem to have some form of back pain.  However this back pain is different.  I now carry 35lbs of books on my back on tuesdays and thursdays, and i weight 105lbs.  I have stabbing sensations on my spine under my bra strap, and low low back pain that shoots thru my left butt check into my leg.  Had an MRI done on my lower back friday and dr said it looked fine, so if its fine then why does it hurt so bad?  Dr gave me muscle relaxer, anti inflammatory, cortisone pill and darvocet.  Sad to say nothing is helping, which really ***** because i am also a mother, wife and full time college student.  I dont have time to hurt like this, and with the pain as bad as it is and these pills arent working, it makes it very hard to study, play with my child and do chores..  What comes after theMRI?  Is there anything the dr. can give me for the pain now until they can figure out what to do next? Please help me, I'm at a loss and this just *****.