it shows your immune system is hard at work. Pus is thousands and thousands of dead white blood cells. They build up at the site where they "gave their all" - the site of the infection.
No, it is not, have her immediately checked by your veternarian, she could be developing a problem which could cause a problem with the birth of her pups.
A chancre is the initial sore or lesion of primary syphilis. It occurs at the site where you were infected.
These sores usually do not hurt and will go away by themselves even without treatment....