Hi,
This sounds like an infection,it could be an abscess,but nothing can be said with surety without medical intervention to determine the cause.
You should definitely consult your doctor or breast specialist and get a proper clinical evaluation done. The pus would need to be send for lab analysis so that your doctor will know what medication you will need to solve this problem.
All the best...
It means you should get it checked by a doctor without delay!
That is the only way to determine whehter it might be an infection, or something far more serious.
Best wishes,
bluebutterfly