Pus formation may be due to re- infection of your root canals or it could be due to some periodontal disease caused by plaque and tartar.  Plz take proper x-rays to find out the cause.  All opinions are based only on your history.  Clinical examination supported by proper x-rays is the wise thing.  Feel free to consult me. 

Hello. This is Dr. Mihir Deshpande By your description, I think you might have undergone a root canal treatment in your teeth which are having the caps.  If not you might need it now. Get an X ray examination done at your nearest Dentist.  He/She will suffer you a root canal or re-root canal treatment for the same which should solve your problem.  You might need new caps to be made after that.