LOWER BACK PAIN. Rid of this problem?

I am a 20 year old student and practice karate . I have been having a lower back pain for nearly a month the exact cause of which is unknown to me. The pain doesn't allow me to bend forward. Earlier i was able to touch my head to my kness at my karate class which i am not able to do at the moment. It feels as if something in my lower back is preventing or restraining me from bending forward. I usually feel the pain while sitting and barely while walking. I had consulted an orthopaedic near my house who had prescribed some painkillers and some medicine to prevent gas- problems. Inspite of taking the prescribed course of medicines regularly , the pain still persists. Could it be joint/vetrbrae related problem or nerve related problem ? What can I do to get rid of this problem ?