I had a root canal done under my bridge on one tooth a week ago but the gum is still sore I'm on penicillin V potassium 500 mg every six hours.?

Get Checked. See the Dentist and get checked. It may be normal healing response depending on the complexity of the problem. If still not resolved after the course of antibiotics, then additional treatment may be required. Often it takes months for the discomfort from pressure to subside, but tissue should resolve once infection clears up. Good Luck.
It is normal. This may be due to a previous infection. If the area was painful before the root canal treatment, it may take a while to heal. Take care.