How come there is a throbbing pain in tooth when i had a root canal done?

Multiple reasons: There are multiple possibilities... From a cracked tooth, to infection, to a dentist missing one canal... Return to your dentist and have them take an x-ray and diagnosis the cause of the pain.

Varies: Even though the nerve in the tooth has been removed the surrounding tissues are still alive. You can still get pain from those areas if there is inflammation or infection. There is also the possibility that the tooth is fractured.

Many Possibilities: Pain can come from infection or trauma. In the case of prior root canal treatment it is possible you have a persistent infection. If the pain is immediately after the treatment it may be traumatic. I suggest you seek the care and advise of the dentist that previously treated you to determine the cause of your specific case. Good luck.