Hsg test, curr. on bactrim (sulfamethoxazole and trimethoprim) 3rd day and i am still having burning and pain behind my bellybutton and pelvic area when i urinate, do i have to go to er?

Yes: It's possible that you are taking an antibiotic that is not effectively treating the bacteria you have. By day #3 you should have some relief from the symptoms of a UTI or cystitis. I would recommend you go back to the er and tell them your history and course. Sometimes your original doctor will have done a culture of urine, sometimes not. That's always helpful info to determine if abx is right.