If you are feeling severe pain after filling and if it persists for quite some time then there might be some problem. If filling was done without properly cleaning the cavities then decay might have started again. Its not common to feel pain after composite filling is done but people feel pain for few days. If you have severe pain you can try out doing root canal as this process involves removing contact of teeth from infected nerve. You don't have to be fearful in doing root canal as this is just another type of treatment like filling. I consulted Dr.David Silberman of dental treatment in Richmond in Texas for my dental implants and root canal as I had missing tooth and severe decayed teeth and from my experience root canal is not a process which involves pain.