Sounds like you have the onset of a staff like infection on your puppy. They are usually located on the belly side or the head. Need to keep puppies very clean and their environment needs to be very clean as well. Also the puppy needs to see a vet so he can drain the infected area and administer anti-biotics, this could indeed be very dangerous for your puppy if not swiftly dealt with. Best of luck.
I just brought a half breed pitbull/ american bull dog it has a bump on the stomach in between the hind leg thats yellow and seems too be puss filled ??? What would it be??