I am so sorry to hear about your pup. I am also very concerned about the symptoms you describe.
IF THIS DOES NOT ANSWER YOUR QUESTION, PLEASE WRITE BACK AND I WILL GIVE YOU 100 MORE REASONS... PLEASE NEUTER YOUR PUPPIES!
The lump you are seeing is most likely an umbilical hernia. It is caused by incomplete closure of the belly prior to birth and is usually quite harmless.