If it hurts when you bite on it then it probably has an infection. This has happened to me after having a tooth filled. Three days later when eating some crisps I nearly went through the roof!
It had got infected during the treatment. Have to say I wasn't very happy as in the end I had to have it pulled..
May not be that and you state you had the treatment a month ago which sounds as though it isn't.
Yes the dentist will be able to tell you what the problem is..