What is wrong with my back?

My lower back has been bothering me for a month now. Just last Friday I threw my back out from doing simple exercises. I couldnt get off the floor. Its uncomfortable to sit for a certain amount of time. Walking seems to be better just a little bit. My hips and top of my thighs ache too. I went to the doctors and he prescribed me with anti inflammatory and muscle relaxer. He wants me to get xrays and to look for a physical therapist. Any idea what could be wrong with my back? I have no idea what I ever did to it that it hurts for so long. It was hurting long before I did simple exercises.